Hi there ​@Heather Knafs. 

Happy to help! Great idea adding birthday information into your customer profiles! 

For your existing customers,  you can either (or do both!):

• Send a campaign to your subscribers asking them to share their bday with you and embed a button in this campaign that will link to their manage preference page  for the list they already exist on. You will need to add a date box into your manage preference form to capture this data, however, once it’s live and you send your campaign, users who are interested should just be able to add in their birthday when they find the page and have this info appended to their profile! 
• Add a form that will will only show to a segment of your existing subscribers without the birthday property on your profile and ask them if they’re interested in sharing their bday with you and if so to click the button in the form, which can take them to the manage preference page as well. 

I’d also recommend adding it to the form you show your new subscribers when you ask for their email and also add it into your welcome flow in case they don’t fill it in. You can set a conditional split in your welcome flow to check if the birthday property is present in their profile and if not, to send them a specialized flow email asking for their birthday info in the same way you set up your campaign for existing customers, which is sending them to the manage preference page. 

Hope this helps! 

-Taylor