Sorry if this has already been answered somewhere, but I can't find it.

You cannot choose to hex someone. It is random.

You can get hexed - at random - from trick or treating on profiles.

It randomly happens to you when you Trick-or-Treat on someone's profile.

I've already been hexed, and managed to get my favorite old hex from last year pokes wolf self

Oh, ok. That makes sense. Thank you very much! ^_^

Arya, you're lucky. Me...not so much.

Yeah, you get hexed at random when you trick or treat at a user profile. Some of the hexes actually end up being funny.