Hi manleysrd,

Currently, you can only add a custom field to your invoice to include your Purchase Order (PO).

Here's how:

1. Click the Gear icon on the upper right corner. 
2. Select Account and Settings.
3. Go to the Sales tab on the left menu.
4. Under Sales form content, click the pencil icon to add a custom field.
5. Tick the box and enter the PO number under the Name field.
   
6. Click Save and Done. 

You'll want to customize an invoice, please check this article for future reference: How To Customize Invoices in QuickBooks Online.

Feel free to let us know if you have any other questions. We'll always make sure to help you.