ClientMessageInspector.cs

using System;
using System.ServiceModel.Dispatcher;
using System.ServiceModel.Channels;
using System.ServiceModel;
using System.IO;

namespace RoyalMail
{
    /// <summary>
    /// Used - https://blogs.msdn.microsoft.com/mohamedg/2012/12/13/adding-http-headers-to-wcf-calls/
    /// </summary>
   public class ClientMessageInspector : IClientMessageInspector
    {
        RoyalMailHeaderCredentials _credentials;

        public void SetCredentials(RoyalMailHeaderCredentials credentials)
        {
            _credentials = credentials;
        }

        public object BeforeSendRequest(ref Message request, IClientChannel channel)
        {
            SetMessagePropertyHeaders(request);

            var guid = Guid.NewGuid();

            RequestMessage requestMessage = new RequestMessage(guid);

            requestMessage.WriteLog(ref request);

            return guid;;
        }

        public void AfterReceiveReply(ref Message reply, object correlationState)
        {
            ResponseMessage responseMessage = new ResponseMessage((Guid)correlationState);

            responseMessage.WriteLog(ref reply);
        }

        private void SetMessagePropertyHeaders(Message request)
        {
            if (_credentials == null) return;

            if (request.Properties.TryGetValue(
                HttpRequestMessageProperty.Name, out object httpRequestMessageObject))
            {
                if (!(httpRequestMessageObject is HttpRequestMessageProperty property))
                    throw new ArgumentNullException(nameof(property));

                SetHeaders(property);
            }
            else
            {
                HttpRequestMessageProperty property = new HttpRequestMessageProperty();

                SetHeaders(property);

                request.Properties.Add(HttpRequestMessageProperty.Name, property);
            }
        }

        private void SetHeaders(HttpRequestMessageProperty property)
        {
            property.Headers["Accept"] = "application/soap+xml";
            property.Headers["X-IBM-Client-Id"] = _credentials.ClientID;
            property.Headers["X-IBM-Client-Secret"] = _credentials.Secret;
        }
    }
}