Ordering a unit to do a custom ability?

So I want to have an ability where a unit can attack another unit and have a custom Thunder Clap or Earthquake type spell be cast on the position of the attacked unit.

I've read these forums about creating a "dummy" unit... I kind of understand that... use the action Create <dummy> unit at position of <attacked monster> or whatever, but then how do I order the newly created unit to execute the custom skill? It only seems I can order him to execute premade skills.

Is there another way to do this? I tried messing around with custom abilities, making it fire off an orb, but that didn't seem to fire since Thunder Clap isn't a unit targetted ability, nor is Earthquake.

Just issue the order of the base ability. This will work.