I wasn't able to be there at park opening today because of a dentist appointment but I did stop in this afternoon for a short while. Unfortunately I can't speak to what they did in the early hours, but the direction that I was given was to check in with a CM beneath two maroon (if I remember correctly, which sometimes I don't) umbrellas they have positioned just to the right of the queue entrance for Anna and Elsa which is where they will be giving out return windows, similar to a FP.
Be aware though, depending on how long they keep this new process (I was under the impression it was a trial and could easily change from day to day or next week to see what works and what doesn't), that the times will book up FAST. I was in the park around ... 11 am and it's pretty understandable all of their return windows were booked for the day.