In Universal Email, you could set the Go To Page to page with the form on it, and pass a URL variable:
open the bindings panel, and create a new binding for the URL variable. Click the plus button select URL Variable. for the name enter "sent" (without the quotes).
then you could type the thank you message on the page, select it and add the WEbAssist Show Region Utility.
Open the server behaviors panel, click the plus button and select WebAssist -> Utilities -> Show region.
click the lightning bolt next to the if setting and select the sent URL variable binding.
set IS to "equal To"
and for the value enter "true" (without quotes)