Epson Perfection V500 - Manual (Page 176)

In the Select an event list, choose the button to which you want to assign a program.
In the Start this program list, choose the program you want to assign. Only compatible
programs appear in the list.
Note:
If you want to be given a choice of programs whenever you press the selected
button, click the Prompt for which program to run button instead. If you want
to disable the button from opening a program, click the Take no action button
instead.
Click OK.