Can we just confirm are you creating the purchase order and customer invoice from an estimate?
You can not add anything in the messages section that will translate however if you have access to custom fields,depending on the version of QuickBooks Online you have (in the cog>account and settings>sales>custom fields>tick on name field>tick whether internal/public>save)you can add a field that will be visible in the estimate and whatever you type in there will also convert to the purchase order when you use the copy to purchase order function on the estimate and the same when you use the copy to invoice function.
No I am not converting an erstiamte to an invoice. I am wanting to add a customer purchase order (for those customers who use them) to a sales invoice template designed in word then imported as a template. I used the <custom1> DZ field but during the matching process as part of the import t of the word document this field is reported as not matched. However, I have subsequently found that if you complete the importing process without resolving this warning then the PO number does appear when you print the invoice. So I think I have resolved the issue myself. As a follow on question is it is possible to set the template up so that the PO heading and PO number only appear if a PO is present.
You can import as many templates as you wish and use the customise option at the bottom of the screen to choose the PO template. You could import two different styles one with a PO number and one without, you can choose the relevant template from the customise option.