As for all trades, you can state what your top 3 posting choices would be. However your career manager will send you to the base where your qualifications will match what is needed. Once in a while, your choice line up with what the military wants. Jackpot!
You can also explain to a certain extent, why you wish to go somewhere or stay in a certain geographic region, but if these wishes are followed (compassionate posting), your career could suffer (slower promotion progression compared to peers, for example.)
Sometimes though, you put in your top 3 choices, and you end up exactly where you want to be. Just don't count on it happening every APS season.
(note I am not an MP myself, but I was married to one prior to joining the forces)