Hello there, @GBE.

Let me provide some insights about customizing the Purchase order template in QuickBooks Online (QBO).

Currently, there are certain limitations on the changes that can be made to the Purchase order template in QBO. 

If you're using QuickBooks Online Advanced, the changes you can make for now are to have customized transaction numbers and messages on the Purchase order.

Here's how:

1. Go to the Gear icon in the upper right-hand. 
2. Choose Account and Settings. 
3. In the left menu, tap Expenses.
4. In the Purchase orders section, select the ✎ icon.
5. Put a check on the Use purchase orders box.
6. Enter titles for the custom fields. 
7. Tap Save, then Done. 

Moreover, I'm adding this helpful article as your reference in creating and editing custom fields in QBO Advanced: Create and edit custom fields in QuickBooks Online Advanced.

Come back to this post if you have further concerns and follow-up questions about customizing purchase orders in QBO. I'll be here to lend a helping hand.