I'll provide the steps you'll need to apply a credit in a Purchase Order in QuickBooks Online (QBO), @Shanahs.

Before anything, please know that Purchase Orders are non-posting transactions therefore, applying credits isn't possible. However, you can convert it to a Bill to achieve your goal. 

Here's how to convert a Purchase Order:

1. Select the Expenses menu.
2. Proceed to the Expenses section and locate the Purchase Order on the list.
3. Click the transaction and use Filter menu to narrow the results if needed.
4. Select Copy to Bill in the top-right corner and fill out the necessary details.
5. Click Save and close.

To apply a Vendor Credit to a Bill:

1. Select the Expenses menu and proceed to the Expenses section.
2. Find the new Bill.
3. Select Make a payment. In the bottom section, ensure that you checked the Vendor Credit you're applying for.
4. Click Save and close.

Additionally, to ensure accurate financial tracking, proper deduction of expenses, and maintaining correct balances in your records, check out this article: Enter vendor credits and refunds in QuickBooks Online.

If you need further assistance with adding credits to a Purchase Order, please let us know by replying below. We're always here to help. Have a good one.