How do I link this to my existing United Airlines account?

If you have an existing United MileagePlus Membership Number, you will be prompted to enter this when activating your card via United Airlines Debit Card App. 

If you don’t have an existing United MileagePlus Membership Number, during the sign up journey, the app will display a link which will redirect you to the United Airlines website where you can sign up for a United MileagePlus Membership. Once you have your United MileagePlus Membership Number; you can resume your sign up journey in the app.