How are SOAP web service namespaces customized

I'm writing a soap web service in asp.net core using soapcore to replace an existing web service. the caller's request xml cannot change because we intend to minimize change on that side. the current request xml looks like

<soapenv:Envelope
    xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/"
    xmlns:ser="http://vendornamespace.com/"
    xmlns:idat="http://schemas.datacontract.org/2004/07/iDataContract">
    <soapenv:Header/>
    <soapenv:Body>
        <ser:ActionRequest>
            <ser:composite>
                <idat:param1>H04</idat:param1>
                <idat:param2>100</idat:param2>
            </ser:composite>
        </ser:PlaceOrder>
    </soapenv:Body>
</soapenv:Envelope>

my web service Interface looks like

public interface INewWebsServices
{
    [OperationContract(Action = "ActionRequest")]
    Task<WebSvcResponseClass> ActionRequest([MessageParameter(Name = "composite")] WebServiceReqactionMethod_A);
}

and my request class looks like

[DataContract (Namespace = "http://schemas.datacontract.org/2004/07/iDataContract", Name ="idat")]
    
[MessageContract()]
[System.Xml.Serialization.XmlTypeAttribute(AnonymousType = true, Namespace = "http://schemas.datacontract.org/2004/07/iDataContract")]
[System.Xml.Serialization.XmlRootAttribute(Namespace = "http://schemas.datacontract.org/2004/07/iDataContract", IsNullable = false)]
public class WebServiceReq
{
    [MessageBodyMember]
    public string param1 { get; set; }

    [MessageBodyMember]
    public string param2 { get; set; }
}

this generates a web service request :

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" 
xmlns:ser="http://vendornamespace.com/">
   <soapenv:Header/>
   <soapenv:Body>
      <ser:PlaceOrder>
         <ser:composite>            
            <ser:param1>?</ser:param1>            
            <ser:param2>?</ser:param2>
         </ser:composite>
      </ser:PlaceOrder>
   </soapenv:Body>
</soapenv:Envelope>

Clearly the idat namespace is missing in the header and is not used in the web request parameters.

How do I modify my WebServiceReq class to include the missing namespace so that SoapCore can de serialize the incoming request properly.

You can try to define WebSvcResponseClass in a namespace iDataContract, also decorate the properties with [DataMember] and not [MessageBodyMember]. Also, you may define your service interface as

public interface INewWebsServices
{

    [System.ServiceModel.OperationContractAttribute(Action = "http://domain/namespace", ]
        Task<iDataContract.WebSvcResponseClass> WebServiceReqactionMethod_A(iDataContract.WebserviceRequestclass composite); 
}

using the namespace generates idat as prefix in the web service request if you try it using soapUI.