Add a new custom control Edit Contribution webpart that would also trigger a save action

Hello, we are currently running kentico 9. We need to be able to add a new save button to the edit contribution webpart. This new save button should still perform the save action, but on top of that it should also perform certain custom validation and It should also have some on click javascript code executed. How could I add a custom control into the edit contribution webpart, so I would have full control over it ( being a custom control) and still trigger the save action from within its code, allowing me to save the document I am currently editing.

Thanks a lot in advance!