Use this where you want to initiallise the spell being ready to be launched.
RegisterForAnimationEvent(game.getplayer(),"BeginWeaponDraw")
This is the event that will take the bow drawing
Event OnAnimationEvent(ObjectReference akSource, string asEventName)if aksource == game.getplayer() && aseventname =="beginWeaponDraw" && (game.getplayer().GetEquippedItemType(0) == 7);cast your spell hereendifEndEvent
Use this where you want your spell to not being ready for the player drawing the bow
UnregisterForAnimationEvent(game.getplayer(), "BeginWeaponDraw")
You may want to define a property as "actor property player auto" and later assign to it the value of game.getplayer(), to use it instead of calling that function, which will make it faster (not really needed in this case, but will help your othe scripts as well as other author's and vanilla ones.