Form pop-up scenario

We have hosted Marketo opt-in form in our external website, currently the form displays when new visitor visits and based on cookie value(if they have accepted cookie) it wont get displayed again for consecutive visits. But now we need  to display the form again and again until visitors filled out the form. we have idea to implement .onSuccess(callback) functionality  Form API but not sure if it works. Can anyone share idea how to implement it ?  

Note: Marketo Munchkin is implemented in the external website.