VRB03KT CAVOK,
At the end of the introductory course you are given a sheet upon which you number the disciplines in order of preference and write a little story on the back about why you should be picked for your first preference. This is then given to the managers who look at company requirements and try to tie it in with your choice. They'll also take into account your skills in specific areas. Not everybody gets what they want but that's to be expected.
I always wanted to go on to the aerodrome course but unfortunately didn't get it and was assigned to area. I was a bit peeved off at the time but I'm loving area now.