You need to get the AA PNR so you can select a seat via the AA website.
You can look up your itinerary on classic.checkmytrip.com and maybe the AA PNR will be mentioned there. Otherwise you have to call BA/AA to get it. I had the same issue (BRU-LHR-LAX-HNL

). BA couldn't see the AA PNR, so I had to call AA and they provided it to me.
If I am not mistaken, AA PNR contains only letters, no numbers.