Question about setting up a filter for my Event Handler

[trashgarbage666]

I'm setting up an OnActivate event handler to fire whenever the player activates another actor. The only example filters I've found have been for the player, specific NPCs, or things within a form list. I'd be fine with making a form list containing every NPC in the game, but in my experience, large lists can make the game stagger a little when the event handler starts up. So as of right now, the first filter isn't set, and my event fires whenever the player activates anything, which isn't exactly ideal.

SetEventHandler "OnActivate" MyOnActivateEventSCRIPT "Second"::PlayerREF;

I've made a few blind attempts, like "First"::IsActor but that obviously didn't compile. Hopefully there's a way to do it!