Activate TCL from a Graphic Display button

I need to activate a (non-Batch related) TCL program from a graphic display button. According to 800xA for MOD 300 manual (3BUR002418-600 A) page 26 "graphic displays can have...target blocks that can intiate a TCL program sequence". Unfortunately, there is no indication of how this is done. Does anyone know how to do this? Many thanks!