Supplier links are registered in the Purchase tab in the Update Part - Purchase or under the same tab in the corresponding procedure in the Inventory module. See the image below.
Supplier Link for part.
Export of Supplier Links
In order for a supplier link to be included in the Print EDI Delivery Schedules one of the alternatives for delivery schedule with EDI must be selected in the Export of supplier links field. For a description of the other alternatives, please see the Supplier Links section in the Help under Update Part - Purchase.