Hi there, TerriFoust24. I can help you add customers with credit opening balances in QuickBooks Desktop (QBDT).
You can add a customer by going to the Customer Center. Then, click the New Customer & Job dropdown. You can also follow the steps provided by FishingForAnswers, which is correct too.
In addition, you can refer to this article to learn how to record paid invoices in QBDT: Take and process payments in QuickBooks Desktop.
Get back to this post if you still have concerns about adding a customer in QBDT. I'm always here to help you.