Just did it in NYC AT&T today (12/06), btw, I am PREPAID/GoPhone user. Some representatives may tell you some bs that only post-paid users can do it, it's not real. I am live on it and it works with my GoPhone.
Moving on, it seems your luck may vary store by store, the one on 52nd w/ Park told me they are having issues and they cannot give me an eSim and to try again on Monday.
I immediately called the store on 56th and Lex and they told me they can do it.
In their defense, everybody is new to this thing, so try to be a bit more patient as all the issues and information is ironed out.
The one on Lex did help but it cost me 20$ ..... at that point I was already tired and busy and since it was a store that was actually doing it, didn't want to miss the opportunity and payed. I know there are people who did it for free or for $5.
Very impotant. iMessage (on Mac also) works with only one phone number at a time. Which sounds normal right ? Problem in my case was that for some reason the iPhone picked the secondary phone (that I did not want to use) as my iMessage line, so I had to deregister that secondary phone number in Apple ID settings on my phone. And then register the new one. A bit of a hassle but once you understand how the process works it takes you 5 minutes.