[XmlRoot("customHeader", Namespace = "http://somecompany.com/webservices/security/2012/topSecret")]
public class customHeader: SoapHeader, IXmlSerializable
{
    public customHeader()
    {
        Actor = "http://schemas.xmlsoap.org/soap/actor/next";
        MustUnderstand = false;
    }

    public System.Xml.Schema.XmlSchema GetSchema()
    {
        return null;
        //throw new NotImplementedException();
    }

    public void ReadXml(XmlReader reader)
    {
        //throw new NotImplementedException();
    }

    public void WriteXml(XmlWriter writer)
    {
        writer.WriteAttributeString("soap:actor", Actor);
        writer.WriteAttributeString("soap:mustUnderstand", MustUnderstand ? "1" : "0");
        writer.WriteRaw("some encrypted data");
        //get it exactly the way you want it in here without mucking with Xml* property attributes for hours on end
        //writer.WriteElement(...);
    }
}

public partial class AuthenticationWS
{
    public SecurityHeader SEC = new Security() { username = "xxxx", password = "xxxx", apikey = "xxxx" };

    protected override XmlWriter GetWriterForMessage(SoapClientMessage message, int bufferSize)
    {
        message.Headers.Add(SEC);
        return base.GetWriterForMessage(message, bufferSize);
    }
}