Free shuttle bus is provided from HKIA to Kowloon Station by showing Airline ticket / boarding pass.

Well, generally speaking, you need to show those things first and then you are allowed to get on the shuttle bus. But about the return trip, I heard from my friends that you can explain your situation to the staff. And make sure you will buy the AE tickets after getting off the bus. In most cases, the staff will let you take it. Good luck!