This article give two code examples on how to create POs using Web Services. There is one class that has two methods. The first method will create a standard PO, the second will create a drop ship PO
The main web service call here is
wsDynamicsGP.CreatePurchaseOrder(PurchaseOrder, context, purchaseOrderCreatePolicy)
This is a complete class - create a class in Visual Studio and drop in the code below, with will run unaltered.