Hello there, @Gorman21.

Yes, I'll gladly help you in showing your client's PO number on sales forms and invoices in QuickBooks Online (QBO).

With the new QBO structure, you need to go to the Custom fields page to add custom fields to your sales forms. This gives you more flexibility to add custom data and insights through detailed reports. To do this, here's how:

1. Go to Settings ⚙ or the Gear icon.
2. Select Custom fields.
3. Click the Add field button.
4. Enter a name in the Name field (for example, PO Number).
5. Select the All Sales forms checkbox.
6. Turn on the Print on form switch for the custom field to appear on printed and delivered forms.
7. Select Save.

You can read this article for more details: Add custom fields to sales forms and purchase orders in QuickBooks Online.

Once you're done, you'll have to manually enter the PO number in the custom field while creating your invoice.

Also, you may want to check out this article as your reference to guide you in personalising and adding specific info to your sales forms in QBO: Customise invoices, quotes, and sales receipts in QuickBooks Online.

Let me know if you have other concerns about customising sales forms and managing income transactions in QBO. I'll gladly help. Take care, and have a great day, @Gorman21.