You'll want to join the free UD Beauty Junkies club in order to get your Urban Decay birthday freebie of free Afterglow.
As per the instructions in the email, you need to “log in to your UD Beauty Junkies account and shop like normal (you must purchase any regularly priced item to redeem this reward). During checkout, click on the box to redeem “Your Birthday Fix” in the “UD BEAUTY JUNKIES” section, then select the Afterglow shade you want in the pop-up window.
And finally, if you're looking for more birthday freebies, then you've come to the right place!