The AA scheme works like most others. Just put your AA number in the booking and let miles credit there. Just go to aa.com and can sign up for an account ( it is free to join )

To enrol in the Platinum Challenge, you need to phone AA in the USA and speak to Aadvantage Customer service and they can enrol you in it. It does cost $250. From start date of challenge, you then have 3 months to earn 10,000 points. SYD-LHR in business class will meet that requirement