I believe you have a few options to achieve this functionality, as the standard Mendix Autonumber will set aside a new value for each object created:
Option 1: Create and manage your own ID number
For this option, you could specify your own attribute and use commit microflows to manage the ID numbers. You leave yourself open to potential issues with concurrency, if two users try to create commit/save a purchase ID at the same time.
Option 2: Use a non-persistable entity to handle everything before you save and commit
Create a non-persistable entity that mirrors your existing PurchaseOrder entity. Create that object and pass it into the PurchaseOrder_NewEdit page. Let the use make changes there, then create and commit the real PurchaseOrder entity when the user clicks save.