No they can only arrange it once you have your flight details. That way disney know how many buses they need etc per number of people arriving/departing.
I would call the reservations number and just ask to be put through. That way any questions will be answered by the CM.
for the pp that asked about return times etc.
The time you are given for return is the bus you should take. Disney do not as a rule let you get a later bus than what you have stated on your notice. You can be offered to take an earlier bus if you are at the stop when one arrives but that is fairly rare as the buses especially at busy times can be full. It's 4hours before your flight that you will be collected. I know that seems like a long time but you might not be the last pick up on the route, traffic issues etc plus security at the airport can take a while so that 4 hours can be eaten up quickly. I know from being at OKW that the bus has been to POR and PORFQ before it gets to OKW and it stops at SSR after that. We got held up at SSR because a couple of people were late for the bus (one of their party was there) they just made it before the driver was about to leave. Harsh I know but he did have a bus with other passengers to consider.
It really is a great service though and takes a lot of the hassle out of getting to your resort. Plus it's free