Add xml prefixes to soap header message internal error

Question

I am at the beginner stage in SOAP services and I need to obtain a SAML assertion from an external source, and then pass it directly into the wsse:Security header of an outgoing WCF/SOAP message.

I am creating a client for a server API I can't change. The reply of the server to my request is:

<env:Body>
    <env:Fault>
        <faultcode>env:Client</faultcode>
        <faultstring>Internal Error (from client)</faultstring>
    </env:Fault>
</env:Body>

I'm not sure what is the cause of the problem but I don't have a XSD to verify the structure of the XML. My client currently produces a XML file with the following structure :

<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
  <s:Header>
    <Security xmlns="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd">
      <saml:Assertion></saml:Assertion>
    </Security>
  </s:Header>
  <s:Body></s:Body>
</s:Envelope>

This is an example of XML from who control the server API:

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/">
  <soapenv:Header>
    <wsse:Security xmlns:wsse="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd">
      <saml:Assertion></saml:Assertion>
    </Security>
  </soapenv:Header>
  <soapenv:Body></soapenv:Body>
</soapenv:Envelope>

I tried to follow this guide to solve my problem https://www.outsystems.com/forums/discussion/74369/how-to-change-add-prefix-to-soap-request/ or other suggestion in StackOverflow but it doesn't work, this is my code:

public void SendRequest(X509Certificate2 certificate, ReperimentoDichiarazionePrecompilataCAFPortClient client)
{
    client.Endpoint.Binding = GetCustomBinding();
    client.ClientCredentials.ClientCertificate.Certificate = certificate;

    client.ClientCredentials.ServiceCertificate.DefaultCertificate = certificate;
    client.Endpoint.Contract.ProtectionLevel = System.Net.Security.ProtectionLevel.Sign;
    ServicePointManager.ServerCertificateValidationCallback = new RemoteCertificateValidationCallback(AcceptAllCertifications);
    ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;
    MessageHeader aMessageHeader = MessageHeader.CreateHeader("Security", "http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd", assertion);

    OperationContext.Current.OutgoingMessageHeaders.Add(aMessageHeader);

    rispostaDichiarazionePrecompilataCAF1 result = await client.InterrogazioneDichiarazionePrecompilataCAFAsync(richiesta);
}

private System.ServiceModel.Channels.Binding GetCustomBinding()
{
    var transpor = new HttpsTransportBindingElement();
    transpor.RequireClientCertificate = true;

    var textMessageEncoding = new TextMessageEncodingBindingElement();
    textMessageEncoding.MessageVersion = MessageVersion.Soap11;

    var version = MessageSecurityVersion.WSSecurity11WSTrustFebruary2005WSSecureConversationFebruary2005WSSecurityPolicy11BasicSecurityProfile10;
    var security = SecurityBindingElement.CreateMutualCertificateBindingElement(version,true);

    return new CustomBinding(security,textMessageEncoding, transpor);
}

I tried to intercept the request and changing with BeforeSendRequest of the class SimpleMessageInspector, but it doesn't work either