We appreciate sending a request to add custom fields on a bill template,
CFO Hire.
As a workaround, you can use the Memo field or the Description column to add some notes in the transaction.
I'll also personally share your product suggestion to our engineers about customising the bill template.
I have articles here about managing bills in QuickBooks Online (QBO):
Just reply to this post if you need anything else with creating bills in QBO. I'm here to help.