If you want to always play your PRIMARY role do this TRICK!
put your secondary role as a role that is most commonly picked. If your role you want is a high priority role then put the secondary as the second highest picked lane. Makes it much harder for the computer to ever give you your secondary role.
Might be obvious to do this but thought I'd share anyways.
oh and if you are duo que with a friend, put your secondary role as what your duo partner put as their primary. if you both do this there is literally no way you will get your secondary role.