How to use the Entity class for creating Orders but also add the products on the order???!!!

Hi Everyone, I manage to create a program that can create an order using the link provided below.
While that is fine, the challenge comes from the new_ordernumber field in the Salesorder entity.
This gets automatically generated on the backend, and I'm not sure how I can reference that value to
be used to create a product via the salesorderdetail entity. I provided the code below, but I'm not sure
how to reference the new_ordernumber from the salesorder entity and use it to create a new record in the 
salesorderdetail entity. Can someone please help? Thanks!
Reference: https://msdn.microsoft.com/en-in/library/gg309731.aspx
foreach (var r in orders)
                    {


                        _service = (IOrganizationService)_serviceProxy;

                        // Instaniate an salesorder object.
                        Entity salesorder = new Entity("salesorder");

                        // Set the required attributes. For salesorder, only the name is required.
                        // See the Entity Metadata topic in the SDK documentatio to determine
                        // which attributes must be set for each entity.
                        salesorder["name"] = r.Name.ToString();
                        salesorder["new_ordernumber"] =

                        // Create an salesorder record named  "Test";
                        _orderId = _service.Create(salesorder);

                        Console.Write("{0} {1} {2} created, ", salesorder.LogicalName, salesorder.Attributes["name"], salesorder.Attributes["new_ordernumber"]);

                        // Create a column set to define which attributes should be retrieved.
                        ColumnSet attributes = new ColumnSet(new string[] { "name", "ownerid" });

                        // Retrieve the salesorder and its name and ownerid attributes.
                        salesorder = _service.Retrieve(salesorder.LogicalName, _orderId, attributes);
                        Console.Write("retrieved, ");

                        // Update the customer attribute.
                        salesorder["new_promotioncode"] = "TEST CODE";



                        // Update the salesorder.
                        _service.Update(salesorder);
                        Console.WriteLine("and updated.");

                        // Delete the salesorder.
                        bool deleteRecords = true;

                        if (promptForDelete)
                        {
                            Console.WriteLine("\nDo you want these entity records deleted? (y/n) [y]: ");
                            String answer = Console.ReadLine();

                            deleteRecords = (answer.StartsWith("y") || answer.StartsWith("Y") || answer == String.Empty);
                        }

                        if (deleteRecords)
                        {
                            _service.Delete("salesorder", _orderId);

                            Console.WriteLine("Entity record(s) have been deleted.");
                        }
                    }



                }
            }

            // Catch any service fault exceptions that Microsoft Dynamics CRM throws.
            catch (FaultException<Microsoft.Xrm.Sdk.OrganizationServiceFault>)
            {
                // You can handle an exception here or pass it back to the calling method.
                throw;
            }
        }
    }
}