I'm flying CMB-DXB-LHR-SFO in a couple months; all in F. I made the reservation on the EK website and am flying EK for the first 2 legs. My LHR to SFO flight has an AA flight number but is on BA metal. I have my seats selected for the EK flights, but not sure who to contact for the LHR - SFO leg.

Do I contact AA and give them my EK confirmation; or contact BA and give them my EK confirmation; or contact EK to see if they can give me a AA or BA confirmation number that I can use on their (AA or BA) website to reserve my seat. Thanks for your help.