Navigate to Data⇒Organisation⇒Supplier

Locate the supplier you wish to amend, using the Search function if required. Double click or click the edit icon to open the supplier record.

Under the finance details you will see Pro-forma Yes or No.  If Yes is selected here, users raising a purchase order on this supplier will be warned that it is a pro-forma account.

To set a supplier as Pro Forma, select Yes then click the Save button.