On your userform, select the commandbutton
In the Properties Window (the window which list a lot of properties of that commandbutton you just selected), look for the item "Acceleartor". In the box next to it, enter the shortcut key you want to assign to the button

For example, you assigned the letter "a" to it
When your userform is loaded, you can press Alt+a to use that commandbutton

Regards
Edwin Ta