Go here, enter your AAdvantage number and click on “already a Citi AAdvantage Cardmember”.
It will take you to another screen where it will say
“Thank you for being a Citi® / AAdvantage® and/or CitiBusiness® / AAdvantage® credit cardmember. To celebrate the 30th anniversary of the AAdvantage® program, the AAdvantage® program would like to thank you with 1,000 AAdvantage® bonus miles.
Eligible Citi® / AAdvantage® and CitiBusiness® / AAdvantage® credit cardmembers who register a valid AAdvantage® number below will receive 1,000 AAdvantage® bonus miles.
The AAdvantage® number you enter must belong to the primary credit cardmember.”
UPDATE 10:48 am – It appears they have removed the “already a Citi AAdvantage Cardmember” button