Thanks for taking the time to post your concern here in the Community, Syanna.

Right now, able to customize the Purchase Order(P.O) isn't available. QuickBooks Online is constantly changing and evolving based largely on the suggestions and requests of users. I encourage you to submit feedback to our Product Development Team for review when determining features to be added in updates. This can be done directly through your account by clicking the Gear icon in the top right corner, then select Feedback.

Here's how:

1. Select the Gear icon at the top, then Feedback.
2. Enter your comments or product suggestions. 
3. Click Next to submit feedback.

For more information about how P.O works in QuickBooks Online (QBO), check out below links:

• Create purchase orders in QuickBooks Online
• Copy an estimate to a purchase order
• Bill created from purchase order has incorrect or higher amount

As always, you can get back to me if you have any additional questions. I'm here for you. Have a great day!

Thank you!

@Syanna 

You may need a 3rd party app to do so.