Whether the form was successfully submitted is confirmed to the user in the standard case with a success message in the user interface. We check if this success message is displayed after sending or not.
It does not matter whether the success message is displayed on the same page or the user is redirected to a separate new page.
Just choose "Page contains text" in the section "Define successful test" and type a text string that appears in the success message on your page.
Screenshot Formspector: Choose type of success feedback