WebTestPluginChangingPutBodyInRequest.cs

using System;
using Microsoft.VisualStudio.TestTools.WebTesting;

namespace YourAppLoadTesting
{
    public class AddCorrelationIdToReportPutRequest: WebTestPlugin
    {
        public string ApplyToRequestsThatContain { get; set; }

        public string BodyStringParam { get; set; }

        public override void PreRequest(object sender, PreRequestEventArgs e)
        {
            if (e.Request.Url.Contains(ApplyToRequestsThatContain) && e.Request.Method == "PUT")
            {
                var requestBody = new StringHttpBody();
                requestBody.ContentType = "application/json; charset=utf-8";
                requestBody.InsertByteOrderMark = false;
                requestBody.BodyString = e.WebTest.Context[BodyStringParam].ToString().Replace("\"correlationId\":null",
                    string.Format("\"correlationId\":\"{0}\\\\0\"", Guid.NewGuid()));
                e.Request.Body = requestBody;
            }
            base.PreRequest(sender, e);
        }
    }
}