How do I redirect a form to an a/b test group thank you landing page?

I have a number of landing pages with forms that redirect to a thank you landing page. I'd like to do some a/b testing on the thank you page. I've created the test group, but when I go into the registration page, I don't see any way to change the form to redirect to the approved test group. I see options for each of the individual approved pages, but selecting either of those options just redirects you to the home page. I saw someone asking in the community about doing this and people said it was possible, so I must be missing something obvious.