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

If you have an existing United Airlines 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 Airlines 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 United Airlines membership. Once you have your United Airlines membership number; you can resume your sign up journey in the app.