Hi there, @hovenden-bigpond.
Yes, you can add the purchase order number on your customers' invoice by adding a custom field. I'm here to walk you through on how to accomplish this.
You'll need to add a custom field on the invoice template then enter the purchase order number manually.
To do that:
On the other hand, aside from this option, you may assign the PO numbers on the Description field of the sales transaction. See this:
For more insights about customizing your sales forms in QuickBooks Online, please see this link: Learn how to personalise your sales forms and add the info that matters most to your business.
I'm also adding here a reference that you can visit regarding emailing your customized sales transactions in QBO, you may visit this article: Learn how to attach files, and email sales forms to your customer within QuickBooks Online.
If you need further help with this topic or anything about QuickBooks, let me know by leaving a comment below. I'm always here to help. Have a good one!