Programming Hookin(?) Question

[SOLAR]

Hello friends here is what i want to do...

There's an app(.exe) say notepad I want to be able to call another program/procedure or function when I click the Bold Button.

In plain terms I think I'm asking how to reprogrammed a button that's in an .exe


I think this is called Hooking or something along those lines.

Thank you for any help

SOLAR