You can not earn AA miles for US Airways flights at this time. Cross earning is expected to start on or around January 7. In the meantime, you can credit the US Airways flight to your US Airways account (open an account if you don't have one) and those US miles will (eventually) merge into your AA miles account as part of merger transition.

Alternatively, since you gave up your seat on AA and were re-booked on US, you could contact AA and request Original Routing Credit (ORC). Whether you'll be successful or not in getting it, I can not say.