Welcome to the Community, @tbols!

To link purchase orders to invoices in QuickBooks Desktop (QBDT), you'll need to convert the purchase order to a bill and make it billable. Then, add the billable expense to the invoice. I'd be glad to guide you through the process.

1. Go to Vendors and select Create Purchase Orders.

2. Choose the Vendor and fill out the form.

3. Click Save and Close.

4. Navigate to Vendors and select Receive Items and Enter Bill.

5. Complete the bill and select your open purchase orders, then click OK.

6. Choose a Customer or Job, tick Billable, then click Save and Close.

7. Go to Customers and select Create Invoices.

8. Choose the Customer or Job, then OK.

9. Click Select the outstanding billable time and costs to add to this invoice. You can also Save this as a preference.

10. Choose the billable items, click OK, then Save and Close.

If you want to track your open purchase orders, refer to the "Step 2: Track your open purchase orders" section of this article: Create purchase orders in QuickBooks Desktop.

Should you have any other concerns besides linking the purchase order to the invoice, feel free to reply below. The Community is always glad to help.