Your eTicket itinerary has the Finnair booking reference, " OPERATED BY FINNAIR /AY 0017O/XXXXXX" You can use that 6-character reference and last name at Finnair Manage booking. There you may be able to change you seat. I'm not 100% sure you can assign seats for free unless you have a status with Oneworld. Finnair charges seat assignments even in business class, so I would imagine this policy is applicable for AA issued tickets and AA codeshares on Finnair.