Matomo (Piwik) differentiates between form submits and form conversions because a form may be submitted several times before all form validation errors are resolved by the user. While form submits are detected automatically, form conversions need to be either configured in the administration or triggered manually in your website or application.
To define when a form is converted, go to “Administration => Forms”. There you can specify for each form some simple rules to let Matomo (Piwik) know when this form is converted.
Alternatively, you can trigger a form conversion directly in your website using the method
trackFormConversion. Learn more about this in the Form Analytics Developer FAQ.