i have found the solution
1)Download the wsdl file (save the web service xml in browser as wdsl file)
2)
SvcUtil.exe <path to the wdsl file>
before calling the web service use
System.Net.ServicePointManager.SecurityProtocol = System.Net.SecurityProtocolType.Tls12;
මේකෙන් අපිට හදල දෙනවා...