add qualtics manual

docs/best_practices/online_studies/qualtrics.md

@@ -8,4 +8,13 @@ nav_order: 1
 
 # Qualtrics
 
-Lorem ipsum...
+1. In your Qualtrics survey, create an `Embedded Data` field (Survey Flow > Add new element > Select Embedded Data) and name it the same as the url parameter that contains the `callbackUrl`. Qualtrics will automatically store the URL parameter in this embedded field.
+   1. If you want to use a different name, [refer to this section](#manually-store-url-parameter).
+2. On the last page of your survey, you can create a link back to the pool using the following HTML snipped:
+
+```html
+<a href="${e://Field/redirectUrl}">Finish survey</a>
+```
+
+### Manually store URL parameter
+TODO