It is not automatic. You need to make sure Marriott Rewards has your birthday on file...apparently they don't ask this...hmmm. Anyway, Erica was very helpful at the MR Customer Service desk and put in a work request for me and my account should be credited within 1-2 business days. In the future, they will send you something via email and I guess you must respond in order to receive this birthday credit.
Make sure they have your birthday on file but I find it strange they have almost all my personal info. Nonetheless, they have it now for the future.
Cheers to all. Phil