Hi Walkaboutdiesel, 

You can definitely customise your invoices to add a field for Purchase Order numbers. The "Custom Field" adds extra fields to sales forms which allows you to select "Internal" to show the field in QuickBooks or select "Public" to show the field on customer forms.

Below is the link to the detailed instructions on how to add this field on your sales forms. 

https://quickbooks.intuit.com/learn-support/en-us/customize-forms/how-to-add-custom-fields-to-invoic...

Hope this helps. Don't hesitate to let us know if you have further questions. Cheers!