How to copy dropdown choice to a field of a thing

Use case: the user is uploading a document. This is assigned to the session number, which is by default the current custom state’s value (a number).

In the upload pop-up: the user sees this in a dropdown. The user may change this number.
This is the setup of the drop down (which works fine, the correct value is displayed in the dropdown):

Now I would like to copy the number the user has chosen to a field of a datatype Document (“gekoppeld aan sessie”) , when the user clicks save. I have setup a workflow which does not function:

It simply does not copy the value. Does anyone know why?