Adding a SOAP header in a generated client with SOAP Interceptor with namespaces

Question

I have a generated client from a wsdl without the possibility to Add custom soap headers. I need a soap header lets say "SOAPFIELD1".

I created a Soap Interceptor

SoapHeaderExtension : SoapExtension

Where I intercept the message

public override void ProcessMessage(SoapMessage message)
{
    if (message.Stage == SoapMessageStage.BeforeSerialize && message is SoapClientMessage)
   {             
      var clientMessage = message as SoapClientMessage;
      AddSoapHeader(clientMessage);
   }
}

The add soap header function is like this:

private void AddSoapHeader(SoapClientMessage message)
{
    var soapHeader = new CustomHeader
    {
        SOAPFIELD1 = "abc",
    };

    message.Headers.Add(soapHeader);
}

Custom Soap Header Class

[System.Xml.Serialization.XmlTypeAttribute(Namespace = "http://www.w3.org/2005/08/addressing")]
[System.Xml.Serialization.XmlRootAttribute("*Test123*", Namespace = "http://www.w3.org/2005/08/addressing", IsNullable = false)]
public class CustomHeader : SoapHeader
{
    [System.Xml.Serialization.XmlTextAttribute()]
    public string SOAPFIELD1 { get; set; }
}

The Header gets updated but instead of

<soap:Header>
          <SOAPFIELD1>123</SOAPFIELD1>
</soap:Header>

I get

<soap:Header>
    **<Test123 xmlns="http://www.w3.org/2005/08/addressing">**
          <SOAPFIELD1>123</SOAPFIELD1>
    **</Test123 >**
</soap:Header>

When I remove the Test123 Tag from my custom SoapHeader I get the class name as a structure

<soap:Header>
    <CustomHeader xmlns="http://www.w3.org/2005/08/addressing">
          <SOAPFIELD1>123</SOAPFIELD1>
    </CustomHeader>
</soap:Header>

How can I add a SOAP header directly under the root element soap:Header. I cannot alter the generated classes from the wsdl so I have to use the interceptor

Thank you in advance!