You've come to the right place for help. I'll be happy to share some insights about creating purchase orders in QBO.
QuickBooks makes it easy to add a purchase order to an existing project and I'll show you how. From the left navigation menu, click Projects > navigate to the project in questions > click on Add to project > select Purchase order. I'll attach a screenshot below for reference:
There you go, now you know how to add a purchase order to a project! In case you have other questions, please contact our support team using this link. :)