CustomPlugin.cs

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

namespace CustomPlugin
{
    [DisplayName("Replace Encoded Slash")]
    [Description("Replaces %2F with %252F to avoid the doubleslash (//) escape.")]
    public class FixFilter : WebTestRequestPlugin
    {
        public FixFilter() { }
        public override void PostRequest(object sender, PostRequestEventArgs e)
        {
            if (e.Request.HasDependentRequests)
            {
                foreach (var item in e.Request.DependentRequests)
                {
                    item.Url = item.Url.Replace("%2F", "%252F");
                }
            }
            base.PostRequest(sender, e);
        }
    }
}