Having pondered this for a few days now, my main suspect for who sent Hook the message is Neal.
Whoever Hook’s been in contact with has to know Neal’s address, (I know Hook’s been there before, but to be able to write out the full address properly, I think someone with knowledge of addresses in our world had to write it, that thing had a zip code on it and everything, and how would Hook even know that a zip code is a thing in order to include it in an address), and had to know that there would be something inside Neal’s apartment that would help convince Emma to believe. The only two people who would be able to know that information are Neal and Rumple, because no one else has been inside Neal’s apartment, (not even Hook himself).
There’s another thread about the theory that someone is watching Emma from a cab as she comes out of the resturant, and there’s also a cab that drives past when MonkeyWalsh is flying up the side of the building too. That person in the cab (if it’s anyone and we’re not just completely over analysing the importance of the cab) could be Neal. It’s more believable to me that Hook’s been able to camp out in NYC for a couple of days if he’s got Neal with him. How did Hook even know there was a zoo in Central Park, or any specific, noteable location to tell her to meet him at, if he didn’t have information from someone who knows the city, because he knows nothing about the city himself.
Hook’s reply to Emma question of, “Why didn’t you tell me it was Neal’s apartment,” would be fitting if he and Neal had had a recent conversation about why Neal couldn’t go himself. I know it’s been implied previously that Hook is aware of at least some details of Neal and Emma’s past, but the scene in the Diner Hook was all, “Sure, give it a go,” but in NYC he was all, “Well DUH you wouldn’t want anything to do with him,” which is a shift in tone, like if he and Neal had actually discussed specifics of why Neal himself couldn’t go to her with her current state of memories.
Neal knows a bit about magic, so it’s believable that he could have learned a bit more, enough to make a memory potion, and that could have been a team effort between them. If Neal could get his hands on that crystal ball in Rumple’s castle again, he’d be able to see Emma and recognise that she’s in NYC, in order to know where Hook had to go to find her.
JMo’s been saying in interviews how Hook and Neal have this relationship and stuff, and it’s sorta rung false IMO because we haven’t seen them as being close at all in present day thus far. But if she was saying that with the knowledge that they’ve been working together, then her comments would make more sense.
In the second Sneak Peek for 313, Hook comments that Neal will turn up, which I think lends credence to the theory that he’s been in contact with him, and therefore knows that he’s alive somewhere.