Editing forms

My WordPress website will have forms that will have field pre-populated with data coming from an external API.
Secondly submitting a form will send the data straight to an external api.

My question is how do i get access to form’s back end so i can specify what happens when the form is submitted.
My plan is to on submit, make call a my own functions. But i want to know how to get access to form code for any plugin