Use "start the effect of an ability" rather than this event, mostly because this one probably won't work as it is intended for channeled spells.
For your actions, you need to create a dummy caster for the owner of triggering unit at the position of your casting unit, give it a short expiration timer (2-3 sec), add the chain lightning ability to it and order it to cast the spell on the target unit of ability being cast.
You'd also want to create a temp variable to store the point where the dummy is created, so you can safely remove it at the end of your trigger to clean memory leaks.