[new solution]
add web service reference with the given address. then use the following code instead of xml.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Text;
using System.Threading.Tasks;
namespace Test_WebSvc
{
class Program
{
static void Main(string[] args)
{
try
{
ServiceReference1.AirServiceClient cl =
new ServiceReference1.AirServiceClient();
cl.Open();
cl.Search(new ServiceReference1.AirSearch()
{
SelectedAirlines = new String[] { "BA" }
});
cl.Close();
}
catch (Exception ex)
{
Console.WriteLine(ex);
}
}
}
}