Yes, even if your store uses a third-party cart or offsite checkout, you can still capture birthdays. There are three options, in order of preference.

If your third-party checkout redirects customers to the Shopify Thank You Page or the Order Status Page, the easiest path is to add the Happy Birthday app block to those pages.

The app block doesn't care which checkout the customer came from. As long as they land on a Shopify post-purchase page, the block will appear.

See Happy Birthday and app block compatible post-purchase pages for setup steps.

If your customers don't pass through any Shopify post-purchase page, point them to your Birthday Sign-up Page instead, a hosted landing page at https://{your-store}/apps/birthday.

Drop the link into your post-purchase email, your store footer, or your social bio. Anyone can use it to join your Birthday Club without placing an order first.

If you use Klaviyo, Mailchimp, Omnisend, Brevo, Privy, or Customer Fields, sign-ups can flow into Happy Birthday automatically, regardless of which checkout you use.
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See the relevant integration article for setup.
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​Klaviyo Integration Basics
​Mailchimp Integration Basics
​Omnisend Integration Basics
​Brevo Integration Basics
​Privy Integration Basics
​Customer Fields Integration Basics

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