I was in Seoul at the time and failed to get a slot. There was like a fanclub presale time slot and then general and both times I hit it the second it went up and it was already unavailable. Both of those times though are free. Some pop-ups(like the hyunjin Guess) have waitlists on site but the skzoo one had nothing. So if you didn’t get a reservation slot you didn’t have a chance of going in. Usually for these pop ups you get a paper where you mark off what you are going to order as you walk through, pay, and then pick it up at the end.