Help calling a trigger...

I wanted to have a trigger called when my ranged unit attack hits a target like how the cold arrow frost takes effect after the arrow hits. I tried various Ability Events, but none that I tried were able to accomplish called actions after (when) the ranged attack struck the target.
Any help would be appreciated, thanks.

The event to use is the specific unit event "unit takes damage". It's a specific unit event, though, so you would have to either dynamically create triggers with this event whenever the hero attacked a target (would require Jass for that, though), or simply have a single trigger and add the events to it for all units, but that sucks if you spawn a lot of new units during the course of one game.