Link your referral page to your Shopify store.

Add your referral program to your Shopify store.

The referral program page lets customers join your referral program and refer your business to their friends. We recommend you add a link to your program page directly on your website menu so that customers will know about it.

Here is how to do it: https://help.shopify.com/en/manual/sell-online/online-store/menus-and-links/editing-menus