I appreciate you for reaching out to us here on the Community page, @techplasuk.
I'll share some steps on how to add the customer order number or transaction number on your invoices/transactions in QuickBooks Online (QBO).
You can just customise your template to add the number on your forms, here's how:
For additional reference about customising form templates in QBO, check out this article: Customise invoices, estimates, and sales receipts in QuickBooks Online.
Add a comment below if you have additional concerns. I'm here to keep helping. Have a great day!
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