Activating tracker when submitting a form via Ajax in Wordpress

When submitting a form in Wordpress via Ajax, we like to track the submission using a Goal conversion.

The following script has been added to the form (in the After Field of Formidable):

So, when submitting the form, I get the console outputs, which show me that the function is executed. But when I look in Piwik, there is nothing.

I have the feeling that this is not the correct way to go, but can’t find another solution. Of course, the tracker is already defined in the footer of the site, and piwik.js is already loaded once. So how do I achieve my goal?