That is standard functionality, so it is located in the help.
In DW, go to the bindings window and add the form from the .html page you have the existing form on. Set the existing form action to the page where you are adding UE.
You can drag and drop the form elements from the bindings window onto the body of the email once they have been added in this way.
Then in UE specify the redirect page as your thank you .html page.