Thank you for joining the thread, @AdriennePlaskett.

Unlike invoices that we can automatically add the transaction number when emailing them to customers, we can't automate the option for purchase orders. For now, you can follow the steps provided by JessT above to add the number on the subject line when on the Send email window. 

On the other hand, QuickBooks consistently changes depending on our customers' needs. You can visit our blog to stay current with the latest QuickBooks news and updates.

Also, consider checking this article for more information about handling purchase orders in QuickBooks Online.

You can always visit us here if you have any other concerns. I'm a few clicks away to help. Have a good day!