Add some extra functions to my form

Hello, I am using a widget containing a html form for making a simple communication with a Java server (getting some results). The form is being used for creating purchase orders. It has 2 fields. On 1st the buyer enters a name and on 2nd enters the amount he wants to pay. Then on submit multiple checks happen and user is redirected (if all good) to the payment link that the Java server is sending back. I want to extend this widget for a specific payment category (coinpayments) to first do some actions and then redirect the user to their website to make the payment.
Also this widget will be imported on multiple sites. So htmls must exist build in.

I need this task done today, so offer only if you can start now.