There are a few ways to do this but the easiest and most direct is to use the OEM Triggers.
For example set OEM Trigger 1 to the port and pin number that you have the button connected to, then go to Config menu and System Hotkeys and enter the OEM code for the function you wish into the box at the side of OEM Trigger 1.
So for example Start is OEM code 1000, enter that and the button will act as a Start button.
Hood