C#: modify scale of a form within a modal subform

I have a simple task: make a modal sub-form which is called by a button click from a main form. Then I input scale data to text field in modal window and I have 3 options: OK, Accept, Cancel.

First and third closes window after confirmation (simple messagebox) and accept changes (if nessesary). Accept button does not close sub form, but accept changes (i.e. scale main window).

So the biggest issue here is how to make such Accept button. I'm very new to these concepts, and right now I have no better idea but pass main form reference to the subform and do my work. But it smells like a poor design to me.