I'm passing document.referrer to GetResponse when a user submits the subscription form. The custom field has been added to the form at runtime as instructed in this document and the "Forward Data" option is on. Upon form submission, the parameters are posted correctly. Problem is, the value doesn't show up anywhere in the user's details in GetResponse, where it would be most useful. Where can I retrieve it?
I'm answering my own question s I've found the solution by trial and error. Not even the support people at GetResponse were able to help.
Apparently, only the custom fields that are actually added to the form designer get recorded. So if you use a custom form (not created with designer), on top of adding the <input> fields in your code, you'll have to create them first (dashboard > contacts > custom fields) and add they'll appear in the designer as well (on the right side).
After that, the passed values will show up in the contact's detail card. To retrieve the values for more than one contact in bulk, you'll have to export the contacts to csv, xml or xls, as support says there is no way to display additional columns on the contacts page. The alternative, if you are looking for a specific value, is to filter by the custom field on the contacts page (left side, 'add custom field').