How to change the footer: Fragebogen -> Impressum und Logo. The textbox allows HTML-Code, so you can use "a href..."-tag to add a link. For example:
<a href="www.test.com">Feedback letter</a>, some other words.
You can use the same code on the last page of the survey. You will find the possibility to change the last page on "Fragebogen zusammenstellen", then scroll all the way to the right.
I hope the menu in the english version is still in german as I saw on a video, so my words make sense.