diff --git a/example.rb b/example.rb
index 456173e..ce237bf 100644
--- a/example.rb
+++ b/example.rb
@@ -3,9 +3,11 @@
 require 'campact_user_service'
 require 'faraday/detailed_logger'
 
+service_host = ENV['SERVICE_HOST'] || 'weact-adapter.staging.campact.de'
+
 def instrument_connection_with_extended_logging(client)
   default_options = {
-    ssl: { verify: true },
+    ssl: { verify: false },
     headers: {
       'Accept' => "application/json;q=0.1",
       'Accept-Charset' => "utf-8",
@@ -29,7 +31,8 @@ def instrument_connection_with_extended_logging(client)
 # Pick which API to connect to
 # 1 for session
 # 2 for user
-puts "Which user service are you going to use?\n\t1) session\n\t2) user"
+# 3 for prefill forms
+puts "Which user service are you going to use?\n\t1) session\n\t2) user\n\t3) prefill forms"
 option = gets.chomp
 
 # Get TOTP credentials
@@ -57,17 +60,25 @@ def instrument_connection_with_extended_logging(client)
     token,
     'campact-staging-session',
     {
-      host: 'weact-adapter.staging.campact.de',
+      host: service_host,
       topt_authorization: {user: username, secret: secret}
     }
   )
 when '2'
   puts "I'll need a user account ID. In practice I won't need this here because it can be derived through the session token"
-  external_account_id = gets.chomp
+  account_id = gets.chomp
   account = CampactUserService.account(
-    external_account_id,
+    account_id,
     {
-      host: 'weact-adapter.staging.campact.de',
+      host: service_host,
+      topt_authorization: {user: username, secret: secret}
+    }
+  )
+when '3'
+  puts "I'll need a user account ID. In practice I won't need this here because it can be derived through the session token"
+  account_id = gets.chomp
+  prefill_forms = CampactUserService.prefill_forms(account_id, {
+      host: service_host,
       topt_authorization: {user: username, secret: secret}
     }
   )
diff --git a/lib/campact_user_service.rb b/lib/campact_user_service.rb
index e30d4ae..c6d7ca0 100644
--- a/lib/campact_user_service.rb
+++ b/lib/campact_user_service.rb
@@ -1,6 +1,7 @@
 require 'campact_user_service/client'
 require 'campact_user_service/session'
 require 'campact_user_service/account'
+require 'campact_user_service/prefill_forms'
 
 module CampactUserService
   class << self
@@ -9,9 +10,14 @@ def session(session_id, session_cookie_name, options)
       CampactUserService::Session.new(client, session_id, session_cookie_name)
     end
 
-    def account(external_account_id, options)
+    def account(account_id, options)
       client = CampactUserService::Client.new(options)
-      CampactUserService::Account.new(client, external_account_id)
+      CampactUserService::Account.new(client, account_id)
+    end
+
+    def prefill_forms(account_id, options)
+      client = CampactUserService::Client.new(options)
+      CampactUserService::PrefillForms.new(client, account_id)
     end
   end
 end
diff --git a/lib/campact_user_service/account.rb b/lib/campact_user_service/account.rb
index 6cab1e9..f2fed95 100644
--- a/lib/campact_user_service/account.rb
+++ b/lib/campact_user_service/account.rb
@@ -2,7 +2,7 @@ module CampactUserService
   class Account
     attr_reader :client, :user_id
 
-    # user_id can either be the user's external_account_id or their email address
+    # user_id can either be the user's account_id or their email address
     def initialize(client, user_id)
       @client = client
       @user_id = user_id
@@ -17,8 +17,13 @@ def subscribed_to_newsletter?
       subscriptions.any? {|s| s['type'] == 'newsletter' }
     end
 
+    def prefill_undecided?
+      prefill = account['prefill_forms_state']
+      prefill.to_s == 'undecided'
+    end
+
     def allow_prefill?
-      prefill = account.dig('preferences', 'prefill_forms')
+      prefill = account['prefill_forms_state']
       prefill.to_s == 'allowed'
     end
 
diff --git a/lib/campact_user_service/client.rb b/lib/campact_user_service/client.rb
index 8410aac..cdb0e2d 100644
--- a/lib/campact_user_service/client.rb
+++ b/lib/campact_user_service/client.rb
@@ -19,10 +19,11 @@ def initialize(options)
 
       @connection = Faraday.new(endpoint, faraday_options) do |faraday|
         faraday.adapter adapter
+        faraday.request :json
       end
     end
 
-    %i(get delete).each do |verb|
+    %i(get delete patch).each do |verb|
       define_method("#{verb}_request") do |path, options={}|
         request(verb, path, options)
       end
@@ -39,6 +40,10 @@ def request(verb, path, options)
           req.headers['Cookie'] = format_cookies(options[:cookies])
         end
 
+        if options.key?(:body)
+          req.body = options[:body]
+        end
+
         if topt_authorization
           req.headers['authorization'] = authorization(topt_authorization)
         end
diff --git a/lib/campact_user_service/prefill_forms.rb b/lib/campact_user_service/prefill_forms.rb
new file mode 100644
index 0000000..68a3319
--- /dev/null
+++ b/lib/campact_user_service/prefill_forms.rb
@@ -0,0 +1,24 @@
+require 'cgi'
+
+module CampactUserService
+  class PrefillForms
+    attr_reader :client, :account_id
+
+    def initialize(client, account_id)
+      @client = client
+      @account_id = account_id
+    end
+
+    def update_prefill_forms(prefill_forms_state:, campaign_slug:, **additional_params)
+      params = {
+        prefill_forms: {
+          state: prefill_forms_state,
+          slug: campaign_slug
+        }.merge(additional_params)
+      }
+
+      path = "/v1/prefill_forms?account_id=#{CGI.escape(account_id.to_s)}"
+      client.patch_request(path, body: params)
+    end
+  end
+end
diff --git a/lib/campact_user_service/session.rb b/lib/campact_user_service/session.rb
index a4209d4..0ff2e08 100644
--- a/lib/campact_user_service/session.rb
+++ b/lib/campact_user_service/session.rb
@@ -8,8 +8,8 @@ def initialize(client, session_id, session_cookie_name)
       @session_cookie_name = session_cookie_name
     end
 
-    def external_account_id
-      session["external_account_id"]
+    def account_id
+      session["account_id"]
     end
 
     def has_soft_login_session?
diff --git a/spec/account_spec.rb b/spec/account_spec.rb
index 559d174..4594fa5 100644
--- a/spec/account_spec.rb
+++ b/spec/account_spec.rb
@@ -165,9 +165,7 @@
     it 'should allow prefilling where the user has opted-in' do
       stub_request(:get, "https://test.com/v1/accounts/#{user_id}")
         .to_return(body: {
-          "preferences": {
-            "prefill_forms": "allowed"
-          }
+          "prefill_forms_state":  "allowed"
         }.to_json)
 
       expect(subject.allow_prefill?).to be_truthy
@@ -176,9 +174,7 @@
     it 'should not allow prefilling where the user has not decided' do
       stub_request(:get, "https://test.com/v1/accounts/#{user_id}")
         .to_return(body: {
-          "preferences": {
-            "prefill_forms": "undecided"
-          }
+          "prefill_forms_state": "undecided"
         }.to_json)
 
       expect(subject.allow_prefill?).to be_falsey
@@ -187,12 +183,39 @@
     it 'should not allow prefilling where the user has opted out' do
       stub_request(:get, "https://test.com/v1/accounts/#{user_id}")
         .to_return(body: {
-          "preferences": {
-            "prefill_forms": "disallowed"
-          }
+          "prefill_forms_state": "forbidden"
         }.to_json)
 
       expect(subject.allow_prefill?).to be_falsey
     end
   end
+
+  describe "#prefill_undecided?" do
+    it 'should be true if the user has not decided' do
+      stub_request(:get, "https://test.com/v1/accounts/#{user_id}")
+        .to_return(body: {
+          "prefill_forms_state": "undecided"
+        }.to_json)
+
+      expect(subject.prefill_undecided?).to be_truthy
+    end
+
+    it 'should be false if the user has opted-in' do
+      stub_request(:get, "https://test.com/v1/accounts/#{user_id}")
+        .to_return(body: {
+          "prefill_forms_state":  "allowed"
+        }.to_json)
+
+      expect(subject.prefill_undecided?).to be_falsey
+    end
+
+    it 'should be false if the user has opted out' do
+      stub_request(:get, "https://test.com/v1/accounts/#{user_id}")
+        .to_return(body: {
+          "prefill_forms_state":  "forbidden"
+        }.to_json)
+
+      expect(subject.prefill_undecided?).to be_falsey
+    end
+  end
 end
diff --git a/spec/campact_user_service_spec.rb b/spec/campact_user_service_spec.rb
index 5093bc1..be11c5a 100644
--- a/spec/campact_user_service_spec.rb
+++ b/spec/campact_user_service_spec.rb
@@ -24,11 +24,11 @@
       options = { foo: 'bar', foo2: 'bar2' }
       client = double
       expect(CampactUserService::Client).to receive(:new).with(options).and_return(client)
-      external_account_id = '189b6864-d58a-4f49-8370-4ae0b854a40e'
+      account_id = '189b6864-d58a-4f49-8370-4ae0b854a40e'
       account_api = double
-      expect(CampactUserService::Account).to receive(:new).with(client, external_account_id).and_return(account_api)
+      expect(CampactUserService::Account).to receive(:new).with(client, account_id).and_return(account_api)
 
-      account = subject.account(external_account_id, options)
+      account = subject.account(account_id, options)
 
       expect(account).to be(account_api)
     end
diff --git a/spec/client_spec.rb b/spec/client_spec.rb
index e40d8cc..c6b6409 100644
--- a/spec/client_spec.rb
+++ b/spec/client_spec.rb
@@ -11,6 +11,10 @@
     }
     let(:faraday_builder) { double(adapter: true) }
 
+    before :each do
+      allow(faraday_builder).to receive(:request).with(:json)
+    end
+
     it 'should initialize connection from options' do
       expect(faraday_builder).to receive(:adapter).with(:an_adapter)
 
@@ -87,7 +91,7 @@
     subject { CampactUserService::Client.new(host: 'demo.campact.de') }
 
     before :each do
-      expect(Faraday).to receive(:new).and_yield(double(adapter: true)).and_return(connection)
+      expect(Faraday).to receive(:new).and_yield(double(adapter: true, request: true)).and_return(connection)
       expect(request_builder).to receive(:options).at_least(:once).and_return(request_builder_options)
       expect(connection).to receive(:get).and_yield(request_builder).and_return(response)
     end
@@ -167,11 +171,49 @@
     subject { CampactUserService::Client.new(host: 'demo.campact.de') }
 
     before :each do
-      expect(Faraday).to receive(:new).and_yield(double(adapter: true)).and_return(connection)
+      expect(Faraday).to receive(:new).and_yield(double(adapter: true, request: true)).and_return(connection)
       expect(request_builder).to receive(:options).at_least(:once).and_return(request_builder_options)
       expect(connection).to receive(:delete).and_yield(request_builder).and_return(response)
     end
 
     include_examples 'error handling'
   end
+
+  describe '#patch_request' do
+    let(:connection) { double }
+    let(:request_builder) { double }
+    let(:request_builder_options) { double }
+    let(:response) { double(body: '') }
+    let(:method_under_test) { :patch_request }
+
+    subject { CampactUserService::Client.new(host: 'demo.campact.de') }
+
+    context 'with faraday stubbed' do
+      before :each do
+        expect(Faraday).to receive(:new).and_yield(double(adapter: true, request: true)).and_return(connection)
+        expect(request_builder).to receive(:options).at_least(:once).and_return(request_builder_options)
+        expect(connection).to receive(:patch).and_yield(request_builder).and_return(response)
+      end
+
+      include_examples 'error handling'
+    end
+
+    context 'with HTTP connection stubbed' do
+      before(:each) do
+        WebMock.disable_net_connect!
+      end
+
+      it 'should send the body included in options' do
+        stub_patch =
+          stub_request(:patch, 'https://demo.campact.de/foo/bar')
+            .with(body: { a_field: 'foo', another_field: 'bar' }.to_json,
+                  headers: { 'Content-Type'=>'application/json' })
+            .to_return(status: 204)
+
+        subject.patch_request('/foo/bar', body: { a_field: 'foo', another_field: 'bar' })
+
+        assert_requested(stub_patch)
+      end
+    end
+  end
 end
diff --git a/spec/prefill_forms_spec.rb b/spec/prefill_forms_spec.rb
new file mode 100644
index 0000000..7ab4213
--- /dev/null
+++ b/spec/prefill_forms_spec.rb
@@ -0,0 +1,76 @@
+require 'spec_helper'
+
+describe CampactUserService::PrefillForms do
+  let(:client) { CampactUserService::Client.new(host: 'test.com') }
+  let(:account_id) { 'abcdef123456fedcba' }
+
+  subject do
+    CampactUserService::PrefillForms.new(client, account_id)
+  end
+
+  before(:each) do
+    WebMock.disable_net_connect!
+  end
+
+  describe '#update_prefill_forms' do
+    it 'should update the prefill forms state with just the state and petition slug' do
+      stub_patch_prefill_forms = stub_request(:patch, "https://test.com/v1/prefill_forms?account_id=#{account_id}")
+        .with(body: {
+          prefill_forms: {
+            state: 'allowed',
+            slug: 'save-the-forest'
+          }
+        })
+        .to_return(status: 204)
+
+      result = subject.update_prefill_forms(prefill_forms_state: 'allowed', campaign_slug: 'save-the-forest')
+
+      assert_requested(stub_patch_prefill_forms)
+      expect(result).to be_truthy
+    end
+
+    it 'should update the prefill forms state including UTM parameters' do
+      utm_params = {
+        utm_campaign: 'a campaign',
+        utm_content: 'a content',
+        utm_medium: 'a medium',
+        utm_source: 'a source',
+        utm_term: 'a term',
+        utm_placement: 'a placement',
+        utm_target: 'a target'
+      }
+      stub_patch_prefill_forms = stub_request(:patch, "https://test.com/v1/prefill_forms?account_id=#{account_id}")
+        .with(body: {
+          prefill_forms: {
+            state: 'allowed',
+            slug: 'save-the-forest'
+          }.merge(utm_params)
+        })
+        .to_return(status: 204)
+
+      subject.update_prefill_forms(prefill_forms_state: 'allowed', campaign_slug: 'save-the-forest', **utm_params)
+
+      assert_requested(stub_patch_prefill_forms)
+    end
+
+    it 'should escape special characters in account_id' do
+      unescaped_account_id = 'user@example.com'
+      escaped_account_id = 'user%40example.com'
+
+      prefill_forms = CampactUserService::PrefillForms.new(client, unescaped_account_id)
+
+      stub_patch_prefill_forms = stub_request(:patch, "https://test.com/v1/prefill_forms?account_id=#{escaped_account_id}")
+        .with(body: {
+          prefill_forms: {
+            state: 'allowed',
+            slug: 'save-the-forest'
+          }
+        })
+        .to_return(status: 204)
+
+      prefill_forms.update_prefill_forms(prefill_forms_state: 'allowed', campaign_slug: 'save-the-forest')
+
+      assert_requested(stub_patch_prefill_forms)
+    end
+  end
+end
diff --git a/spec/session_spec.rb b/spec/session_spec.rb
index 5749860..511dc35 100644
--- a/spec/session_spec.rb
+++ b/spec/session_spec.rb
@@ -7,13 +7,13 @@
 
   subject { CampactUserService::Session.new(client, session_id, session_cookie_name) }
 
-  describe '#external_account_id' do
+  describe '#account_id' do
     it 'should be present' do
       stub_request(:get, 'https://test.com/v1/sessions')
         .with(headers: {'Cookie' => "cus-session=#{session_id};"})
-        .to_return(body: {external_account_id: '189b6864-d58a-4f49-8370-4ae0b854a40e'}.to_json)
+        .to_return(body: {account_id: '189b6864-d58a-4f49-8370-4ae0b854a40e'}.to_json)
 
-      expect(subject.external_account_id).to eq('189b6864-d58a-4f49-8370-4ae0b854a40e')
+      expect(subject.account_id).to eq('189b6864-d58a-4f49-8370-4ae0b854a40e')
     end
   end