How do i dedicate a button to a desired function under Start/Settings/Buttons?

[discipler]

I hesitate to ask because the question has probably been answered on these pages a number of times. But through my searches i have been unable to find the answer. I loaded some software that called for dedicating a hard button to activate it. Going to Start/Settings/Buttons i could not figure out how to do it. The program is wmsnapshot and when the dedicated key is pushed, a snapshot of the page is taken. Can someone help?