Using a subcontract routing

We would like to start using the subcontract functionality to send material out to sterilization. From what I understand we would use the routing for this purpose by selecting external processing in the routing and selecting the subcontract button. Doing this however, does not allow the creation of purchase Req. in the production order. It automatically sets the field "res/Purchase Reqs" in the external processing tab to "1" which is never create Purchase req. If the subcontract field is not selected it allows purchase reqs. Am I missing some other setting that is required to make this work?