Where and how to use Hong Kong Airport Express ticket going to airport?

I understand we are priviliged to this City Check-In either at Kowloon or Hongkong station (because our airlines is Cathay Pacific) provided we will buy the airport express single journey ticket going to the airport which costs around HK$110 and they're going to collect already our luggages with no extra costs and we will be issued already the boarding pass. Question is, before entering the Kowloon station for buying the airport express ticket, are we allowed to enter without using any card because remember we're still going to buy and I understand the ticketing booth is inside the station. After buying, that's the only time we can proceed on the city check in, am I correct? And can we leave the station freely without using the card? because remember the airport express single journey we bought is supposed to be used later when we are going to the airport already. Please help me on this. thanks.

mfemylah