If the purchase order is related to a project or a WBS project, you can create this relationship under the "Accounting" tab -> "Project" tab of the purchase order.
For this you need to select Project from the "Projects" window, and if you later want to define a WBS Project , you can select it from the "Project Tasks" window. By saving and sending them, you can link them to the document.
If you want to associate more projects, after selecting the first one, you should type "% Rate" and save. The system will show another two-row field to add more "Projects" and "WBS Elements" and will show the remaining % rate to complete 100%.
To delete project records, you must check the "Delete" checkbox and click the "Delete" button.