You entered the code when you initially registered on the site.
However, at the end of the month, I go through subscriptions for that month and if the referrer code field is blank, I check to see if they're posting on this board or if they mentioned Zeev in the "Who referred you" field. If either of those conditions are true, I put "ZH" in the appropriate field. If neither of them is true, I put "BZ" in there so a portion of their subscription will be paid to my favorite charity.