How to save the referring page in a form

Your software
My Mautic version is:2.15
My PHP version is:7.0

Your problem
My problem is:
I have tried to implement the technique described here: https://mautic.com/help/save-referring-page-form/
However the ‘ref’ field in the form is not being populated. My form is contained in a full page pop-up focus item which triggers to unidentified visitors on any of our website pages after 40 seconds. So it is not a form fixed to certain page. Perhaps this could be he issue… although it shouldn’t.

These errors (lots of them - identical) are showing in the log (The y have been showing for a while now and I did not paid much attention to them):
AH01071: Got error 'PHP message: PHP Notice: Undefined variable: submissions in /home/…/app/bundles/FormBundle/Views/Builder/form.html.php on line 59\nPHP

Any suggestions on why the implementtation is not working? Thank you.