Note: Before you can set a prepayment on an item, you must select a prepayment hierarchy in the Customer Order Setup page, see Step 3 in How to: Set Up Vendor Sourcing for Customer Order.
This is a regular item hierarchy that decides what items can be Vendor Sourced.
Items can be added manually or with the Add Items action.
On the hierarchy that is set as Prepayment Hierarchy, the Prepayment % field is visible. The default value is Prepayment % = 0 and it can be modified on Hierarchy Nodes and Hierarchy Links.