Thanks for asking about this, JH72.
QuickBooks Online (QBO) allows customization only for sales forms, such as invoice, sales receipt and estimate. This doesn't include the Purchase Orders. The description in your PO is associated with the category or product/service you've entered in the transaction.
You'll want to visit this article to learn more about the available sales forms that you can customize in QBO: Customize invoices, estimates, and sales receipts in QBO.
For now, I want you to be updated with our latest happenings, feature developments through our QuickBooks Blog.
Should you be needing more help from me, just leave a reply below. I'd be more than happy to assist. Have a great rest of the day!