The process I have settled on:

1. Make a reservation, not logging in with my AA number.

2. Call AA, have phone agent add your AS number.

3. Purchase reservation, ask for a mailed receipt.

4. When you get your receipt, check each flight to make sure the AS number is there sometimes they will print your mileage number for each flight. I once logged in to AA.com with AA number, then called to add AS number, when the receipt arrived, AS number on flights 1,2,3 however, flight 4 had AA number.

keep all boarding passes and receipt, you will probably need them. have had 3 trips so far that did not auto credit AS.