You've come to the right place for answers, @garyjhogg-hotmai. I hope you're good and well today.

I'll ensure that you can add a purchase order (PO) number to your invoice in QuickBooks Online (QBO). This way, you can easily track them anytime. 

Let's create a custom field for PO number to achieve your goal. I'm happy to provide the detailed steps on this. 

1. Go to the Gear icon, then choose the Custom fields. 
2. Click the Add custom field button. 
3. Enter PO Number in the field and check the box beside All Sales forms
4. Turn on the radio button in Print on form (optional). 
5. Hit Save.

For more information, check out this link: How to add custom fields to sales forms and purchase orders.

After that, open the invoice and enter the desired PO number in the field. See the sample screenshot below for your reference.

In addition, see these articles below about how to copy purchase orders to quote and customise sales forms:

• Copy a quote to a purchase order
• Customise invoices, quotes, and sales receipts in QuickBooks Online

I'm always here to listen and help if you have other QuickBooks concerns. Just post a reply below. Keep safe, garyjhogg-hotmai.