PortTypeClient mservice = new PortTypeClient();
string mlogin = mservice.login("user", "password");

<?xml version="1.0" encoding="utf-8"?>
<configuration>
    <configSections>
    </configSections>
    <system.serviceModel>
        <bindings>
            <basicHttpBinding>
                <binding name="BTVBinding" maxReceivedMessageSize="2000000">
                    <security mode="Transport" />
                </binding>
            </basicHttpBinding>
        </bindings>
        <client>
          <endpoint address="https://admin.mywebsite.com/index.php/api/v2_soap/index"
              binding="basicHttpBinding" bindingConfiguration="BTVBinding"
              contract="BTVMagento.PortType" name="Port1" >
          </endpoint>
        </client>
    </system.serviceModel>

System.ServiceModel.FaultException: 'SOAP-ERROR: Parsing WSDL: Couldn't load from 'https://admin.mywebsite.com/index.php/api/v2_soap/index/?wsdl=1' : Premature end of data in tag html line 7