Hello there, @LindsayB1. I'm here to help you add interest on past-due accounts in QuickBooks Desktop (QBDT). 

The option to automatically add interest on past-due accounts in QBDT is unavailable. You'll have to manually add it. To do so, you can create a service item for the interest and add it to the invoice. Here's how:

1. Navigate to the Lists menu and select the Item list.
2. From there, select the Item dropdown arrow and choose New. 
3. In the Type section, select Service. 
4. Next, enter the name of the interest and add the needed information.
5. Then, choose the correct account where you want to track the interest. If you haven't created one, just click Add new.
6. Once done, select OK. 

To ensure the item is assigned to the correct account, I recommend asking for assistance from your accountant. They can give proper advice on which account to use and how to track the interests. If you're not affiliated with one, you can use our Find an Accountant tool to look for one near you. 

I'll include this helpful resource that you can use as a guide in helping you to easily identify past due invoices in your account: Show Past Due stamp on invoices in QuickBooks Desktop. 

I'm always here ready to lend a hand if you have any other concerns about managing past-due accounts in QBDT. Have a good day ahead and keep safe.