If you booked with a travel agent and want service, then the TA would be the point of contact. If you want to use an AA agent then AA has fees for doing actions on the agent's behalf

In this case, you may be able to assign seats for the Qantas flight on the Qantas site. If you go to Qantas.com and use its manage booking facility, you may well be able to select seats for free) . Alternatively go back to your TA