Really really easy:
1. Find the classname of the weapon (should start with rkt, check the config to find out, or use hint format ["%1",weapons this] in the init box of the plane and it will show you the weapon names in a hint box at the mission start)
2. A6 fire "classname" A6 = the name of your plane and "classname = the weapon classname.
Should work, try it out.