public class SelligentOrganizationToken
    {
        public string organizationName { get; set; }
        public SelligentOrganization subOrganizationId { get; set; }
    }

    public class SelligentOrganization
    {
        public string id { get; set; }
    }

    private MailingService CreateMailingService(string mailingServiceUrl, string userName, string password)
    {

        var securityElement = SecurityBindingElement.CreateUserNameOverTransportBindingElement();
        securityElement.IncludeTimestamp = false;
        var encodingElement = new TextMessageEncodingBindingElement(MessageVersion.Soap11, Encoding.UTF8);
        var transportElement = new HttpsTransportBindingElement();

        var customBinding = new CustomBinding(securityElement, encodingElement, transportElement);

        var remoteAddress = new EndpointAddress(mailingServiceUrl);

        var mailingService = new MailingServiceClient(customBinding, remoteAddress);

        mailingService.ClientCredentials.UserName.UserName = userName;
        mailingService.ClientCredentials.UserName.Password = password;

        ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls11;

        var selligentOrganizationToken = new SelligentOrganizationToken()
        {
            organizationName = "admin",
            subOrganizationId = new SelligentOrganization()
            {
                id = "1"
            }
        };

        //Serialize object to xml
        XmlObjectSerializer xmlObjectSerializer = new DataContractSerializer(typeof(SelligentOrganizationToken), "OrganizationToken", "http://www.strongmail.com/services/v2/schema");

        var eab = new EndpointAddressBuilder(mailingService.Endpoint.Address);
        eab.Headers.Add(AddressHeader.CreateAddressHeader("OrganizationToken", "http://www.strongmail.com/services/v2/schema", selligentOrganizationToken, xmlObjectSerializer));
        mailingService.Endpoint.Address = eab.ToEndpointAddress();

        return mailingService;
    }