<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:ser="http://services.moscow.com/">
   <soapenv:Header/>
   <soapenv:Body>
      <ser:ConnectToScheme>
         <!--Optional:-->
         <ser:login>1</ser:login>
         <!--Optional:-->
         <ser:pwd>1</ser:pwd>
      </ser:ConnectToScheme>
   </soapenv:Body>
</soapenv:Envelope>

<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
   <soap:Body>
      <ConnectToSchemeResponse xmlns="http://services.moscow.com/">
         <ConnectToSchemeResult>false</ConnectToSchemeResult>
         <errStr>Ошибка при подключении к схеме: Пользователь 1 не найден</errStr>
      </ConnectToSchemeResponse>
   </soap:Body>
</soap:Envelope>

var doc = new XmlDocument();
            doc.Load("request.xml"); // тот замый запрос, взятый из SoapUI
            var request = (HttpWebRequest)WebRequest.Create("http://localhost:4690/Webservice_Moscow/PlaningService.asmx");
            request.Accept = "text/xml";
            request.Method = "POST";
            request.Headers.Add("SOAPAction: 'http://services.moscow.com/ConnectToScheme'");
            request.ProtocolVersion = HttpVersion.Version11;
            request.Credentials=CredentialCache.DefaultCredentials;

            using (Stream srtm = request.GetRequestStream())
            {
                using (var stmw = new StreamWriter(srtm))
                {
                    stmw.Write(doc.InnerXml);
                }
            }

            var response = request.GetResponse();