index.html

<!doctype html>
<html> 
<head>
  <title>GitHub Authentication Example</title>

  <!-- Make sure to include Nelify's authentiation library -->
  <!-- Also available via npm as netlify-auth-providers -->
  <script src="https://unpkg.com/netlify-auth-providers"></script>
</head>
<body>
  <h1>GitHub Auth Demo:</h1>
  <p><a href="#" id="login">Authenticate</a></p>
  <p id="output"></p>
  <script>
    const anchorTag = document.getElementById('login')
    const outputText = document.getElementById('output')

    anchorTag.addEventListener('click', (e) => {
      e.preventDefault()

      const authenticator = new netlify.default ({})

      authenticator.authenticate({provider:"github", scope: "user"}, (err, data) => {
        err ? outputText.innerText = "Error Authenticating with GitHub: " + err : 
        outputText.innerText = "Authenticated with GitHub. Access Token: " + data.token
      })
    })
  </script>
</body>
</html>