//string endpointUri = string.Format("https://{0}/adfs/services/trust/13/usernamemixed", _serverName);
        string endpointUri = string.Format("https://{0}/adfs/services/trust/2005/usernamemixed", _serverName);

        var factory = new WSTrustChannelFactory(
                      new UserNameWSTrustBinding(),
                      new EndpointAddress(endpointUri));

        //factory.TrustVersion = TrustVersion.WSTrust13;
        factory.TrustVersion = TrustVersion.WSTrustFeb2005;

        if (factory.Credentials != null)
        {
            factory.Credentials.UserName.UserName = _userName;
            factory.Credentials.UserName.Password = _password;
        }

        var rst = new RequestSecurityToken
        {
            RequestType = RequestTypes.Issue,
            KeyType = KeyTypes.Bearer,
            AppliesTo = new EndpointReference(_relyingPartyUri),
            KeySizeInBits = 0,

            TokenType = "urn:ietf:params:oauth:token-type:jwt",

        };

        var channel = factory.CreateChannel();
        try
        {
            var token = channel.Issue(rst);
            return token as GenericXmlSecurityToken;
        }
        catch (ProtocolException ex)
        {
            Debug.Write(ex.Message);
        }
        return null;
    }