How do you say hit me up in Spanish?

It depends on how you mean "hit me up".

If you want the other person to "hit you up" in slang-Word the sense of "calling you" or "finding you", then: "Llámame" OR "Échame un grito" which mean literally: "Call me" OR "Send me a shout."

If you want the other person to physically assault you (i.e. to get hit/punched), then "Golpéame" which literally means "Punch me."