public class TicketHeader : System.Web.Services.Protocols.SoapHeader
{
    public string Content;
}

[WebService(Namespace = "http://localhost/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
public class MainService : System.Web.Services.WebService
{
    public TicketHeader Ticket;
    [WebMethod(EnableSession = true)]
    [SoapHeader("Ticket", Direction = SoapHeaderDirection.In)]
    public string Check()
    {
        ......;
    }
}

var service = new MainServiceSoapClient();
var ticket = new TicketHeader { Content = "....." };
// <---- тут нужно бобавить заголовок
var result = service.Check();