You can't add a BA Exec number to an AA booking on-line. You either have to call up AA or (as I usually do), get the number added at the airport at check-in.
As a OW Emerald you have access to the Admirals lounge, which is reasonably pleasant. Don't try going to the BA lounge - it's in a different terminal. The Admirals lounge staff can also add the BA Exec number to your booking - useful if the the checkin area is busy - and my experience with them is that they are helpful and cooperative.