By enabling this setting, salespeople must be aware when entering a valid value in the Customer PO # field in the following screens: Standings Orders, Prebooks, and Order Entry.
To enable this field as required for a particular customer, please follow these steps:
Go to Setup > Customers.
Select the desired customer and click on Edit link under Actions column.
Confirm the checkbox is unchecked, check it and save the changes.
As soon you create an order in any of the screens mentioned at the beginning of the article, the Customer PO # field will require any valid information in order to continue with the process.