You can't detect when autocast attack-based abilities fire, and even if you could, that would be when the attack started, not when the arrow actually hit.
How I did it was a sort of a work around. I used an arrow ability that creats a buff on the target (not black arrow, because it can't affect heroes, but either cold or frost arrow; one of them allows the use of custom buffs, the other one doesn't.) when it hits. Then I had a variable (variable array, actualy, so I could have more heroes of the same type) that stored the last unit my hero attacked, and I had a fast periodic trigger (running every 0.04 seconds) that checked those units for the arrow's buff. Because the periodic trigger was so fast, the delay between the arrow hitting the target and the trigger noticing the buff was unnoticable. The only problem you would get this way is, you don't know who the attacker was if two same heroes were targeting the same unit; The trigger would check the first hero's target, find the buff and assume it was the first hero that inflicted it, even if it was the second one. For me, it didn't really matter, because the effect I was applying was negative armour, and it really didn't matter to which player the dummy caster that cast the armor reduction spell belonged.
You could use a "unit takes damage" event instead of a periodic one, that is even more accurate than a periodic trigger, and you know who dealt the damage that created the buff, but since this is a specific unit event, it's hard to use without Jass, because if you are creating and killing a lot of units in your map, you have to add a new "unit takes damage" event to a trigger for each unit you make, and you can't remove events, so this can eventually cause lag (in Jass, you can just dynamicaly create a trigger for each unit, and you can remove the whole trigger when the unit dies). | 
JASS: