When a customer joins your Birthday Club after a purchase, typically via the Happy Birthday app block on your Thank You or Order Status page, Happy Birthday tries to attach the birthdate to that customer's record in the Customer Fields app.

For that to work, Customer Fields needs to be set to import customers from Shopify automatically.

In the Customer Fields app, go to Settings → Sync Settings. Make sure the option to automatically import customers from Shopify when they place an order is turned on.

Once enabled, every post-purchase Birthday sign-up will be pushed into Customer Fields and used to update the customer's record with their birthdate.

Please get in touch with support if you need any further assistance.