public class CreateExtension : IEndpointBehaviorExtender
    {
        #region IEndpointBehaviorExtender Members

        public string Action
        {
            get { return "Expense/Create"; }
        }

        public void BeforeSendRequest(string action, ref Message request, IDictionary<string, object> formData)
        {
            // create a strongly type version of the message
            ExpenseCreateRequestMessage req = MessageConverter<ExpenseCreateRequestMessage>.GetObjectFromMessage(action, request);

            // the data contract already has additonal collections (dictionaries) so that 
            // additional data can be stored along with the default contract.

            // Try to make 'key' as unique as possible so that only you could identify it. Perhaps add your name / intials or BP Channel Name
            if (!req.Expense.FieldExtensions.ContainsKey("RBC_DateExpenseCreated"))
                req.Expense.FieldExtensions.Add("RBC_DateExpenseCreated", DateTime.Now.ToLongDateString());

            // replace the message request with new one
            request = MessageConverter<ExpenseCreateRequestMessage>.SetMessageFromObject(action, req, request.Headers.MessageVersion);
        }

        public void AfterReceiveReply(string action, Message reply)
        {
            // convert the message to its strong type (as described in contract library)
            ExpenseCreateResponseMessage response = MessageConverter<ExpenseCreateResponseMessage>.GetObjectFromMessage(action, reply);

            // do stuff with respones...
        }

        #endregion
    }

Web Extensibility Download Part 3 - Extending Client-side code behavior

C#