You can display a message that form submission has been successful on the very first form page without taking user to a Thank You page after submission. For this you need to do the following.
Add this code to the form source to the place where you wish to display the message:
Perhaps, you would like to add it right next to <!-- FPP_ERROR -->.
Then you need to add the message itself to the config.php:
As you can see, you can use HTML code in your message.
You can set this option globally for all forms (by adding it within the global settings section at the top of the config.php) or locally for a particular form (if you add it to the form settings section).
Then duplicate the last 'page' entry (it is usually the only entry for this kind of one-page forms) in the config.php to tell FPP that it needs to stay on that page. So, if you had this line pointing to:
made it double:
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