To select project while creating Purchase order, please follow the below mentioned steps:
Step 1: “Create a Project”.
Please follow the steps mentioned in the below article to create a project.
Step 2: Go to “Purchases” >> Click on “Purchase order” >> Click on “Create Purchase order".
Step 3: Click on “Project” >> “Select Project” >> “Select Vendor” >> “Add Products” >> Click on “Save”.

