British Airways - where can I input Alaska mileage plan?
 

Booked Premium Economy Seattle to Stockholm; no obvious place on the Manage Bookings UI to enter my AS mileage plan number. Do I do this at the airport?

You can do it that way and you have to pad your AS number with three leading zeros. The agent might not know that.

I have found BA routinely does not credit back to AS even when the number is in the reso; would recommend you just submit the BPs afterwards if you want credit.

And I recommend bringing both a copy of your card and a printout of the relevant page from the AS website describing Galleries lounge access policy. At LHR T5 last week I had the whole "this card doesn't get you into the lounge"..."i have never heard of alaska, they arent on this [points to one world sign] list..." handed her the sheet, she looked up "exceptions" on the computer and voila.

Look up your reservation on Finnair - then you should be able to input your MP number.