In the app while on the booking, go to “Manage trip”. Then scroll down to “Passenger information”. Here you can add KTN. Not sure if your BA number is already entered on this type of booking. If not, you can just add your AAdvantage number higher up on the same screen. If it already has your BA number, then I think this requires a call.

Edit: was a bit slow. But I think if OP has already booked, then too late to book for someone else.