Let me share some information about customizing template in QuickBooks Online (QBO).
This time, you can only customize sales forms, such as invoice, sales receipt, and estimate in QBO. Changing the Purchase Order (PO) template is currently unavailable.
It's our priority to provide products and services that suit your business needs. I'll make sure to take note of this and will inform you once we received product enhancement update about the availability of this feature.
While we don't have a timeline for when this will be available, you can check out our blog site. This is the place where we share information if there's a feature update.