Bill Button in Purchase Order

i have a requirement to automate logic in the BILL button displayed in purchase order form. i have tried the below code

        CashSale cSale = new CashSale();
        cSale.createdFrom = new RecordRef { internalId = "XXXXXXXX", type = RecordType.salesOrder, typeSpecified = true };
        cSale.entity = new RecordRef { internalId = "XXXXXX", type = RecordType.customer, typeSpecified = true };
        cSale.tranDate = System.DateTime.Now;
        cSale.location = new RecordRef { internalId = "1", name = "XXXX", type = RecordType.location, typeSpecified = true };
        string postingPeriod = String.Format("{0:MMMM}", DateTime.Now) + " " + DateTime.Now.Year;
        cSale.postingPeriod = new RecordRef { name = postingPeriod};  
        CashSaleItemList cSaleItemList = new CashSaleItemList();
        CashSaleItem[] cSaleItems = new CashSaleItem[1];
        CashSaleItem csItem = new CashSaleItem();
        RecordRef roItem = new RecordRef();
        roItem.type = RecordType.inventoryItem;
        roItem.typeSpecified = true;
        roItem.internalId = "XXXXX";
        csItem.item = roItem;
        RecordRef prLevel = new RecordRef();
        prLevel.type = RecordType.priceLevel;
        prLevel.internalId = "-1";
        prLevel.typeSpecified = true;
        csItem.price = prLevel;
        csItem.rate = "15";
        csItem.quantity = 1;
        csItem.quantitySpecified = true;
        InventoryAssignment iAssignment = new InventoryAssignment
        {
            binNumber = new RecordRef { internalId = "XXXX", type = RecordType.bin, typeSpecified = true },
            quantity = 1,
            quantitySpecified = true
        };
        List<InventoryAssignment> iAssignmentList = new List<InventoryAssignment>();
        iAssignmentList.Add(iAssignment);
        csItem.inventoryDetail = new InventoryDetail
        {
            inventoryAssignmentList = new InventoryAssignmentList
            {
                inventoryAssignment = iAssignmentList.ToArray()
            }
        };
        cSaleItems[0] = csItem;
        cSaleItemList.item = cSaleItems;
        cSale.itemList = cSaleItemList;
        WriteResponse response = Service.add(cSale);
        if (response.status.isSuccess != true) throw new Exception(response.status.statusDetail[0].message);

This creates record of type Cash Sale and link type Order Bill/Invoice. But i want to create record of type Bill and link type Order Bill/Invoice.

Searched for the C# code. can’t find it.Can anyone provide me a code in C# to perform the same action as BILL button in the purchase order form?

Thanks in Advance.