Hey there, @warren6.

I'll be glad to assist you in entering your client's purchase order number on your invoice.

You can use the Message on invoice field to enter this detail in your invoice. You can also use this field if you wanted to add more info to your sales transaction. Let me show you how:

1. Click the +New icon and select Invoice.
2. Choose the name of your client in the Customer drop-down list.
3. From the Product/Service column, select a product or service and enter a quantity, rate, and change amount if needed.
4. In the Message on invoice field, add the purchase order number. 
5. When you're ready to send the invoice, select Save and send. 

Please take note that before you get paid, QuickBooks puts open invoices into your Accounts Receivable account. You'll see this account on your Balance Sheet and other financial reports.

In case you need to inform your customers about their upcoming invoice due dates, here's how to send them a reminder message.

You're more than welcome to visit this thread again if you have other questions. Just add the details of your concern here and I'll help you out.