A bill was created. how do I add the PO to it afterward?

I'd be glad to provide details about your concern, bayer368.

A purchase order is a non-posting transaction, and adding it after a bill is created is unavailable. What you can do is create a purchase order first and then copy it to the bill. 

Here's how:

1. Go to the + New > Purchase order. 
2. In the Purchase order page, select the vendor from the drop-down menu.
3. Add the Category or Item details.
4. Click Save and you will see the Copy to Bill button beside the amount. 
5. In the Bill page, click the Save and close or Save and schedule payment button.

For more details about the process, you can check out these articles:

• Create purchase orders in QuickBooks Online
• Enter and pay bills

Please feel free to leave a reply on this thread if you have other questions or concerns. I'll be around to help you out again. Have a good day!