xmodmap

.Xmodmap_src

Content Explaination

keycode 156 = XF86Launch1
add mod3 = XF86Launch1

line 1: Assign XF86Launch1 (X11 treat KEY_PROG1 as XF86Launch1) symbol to keycode 156 (KEY_COMPOSE in my case) line 2: Add XF86Launch1 to mod3