Sure, you can do this in 2 ways:
1. In an onSuccess listener, dynamically choose the Thank You based on the current URL (for example, appending "_ThankYou" to the current page).
2. In the form setup, by using Add Choice on the Settings page.
I can't actually see the referrer URL as an option you can select when setting the choice for the thank you page. It is only offering the fields/questions I have on the form. Can you outline how you would do this? Am I missing something?