Show ProgressBar with percentage while uploading in WP8

Question

Here is my code for uploading,

 byte[] buffer = PhoneApplicationService.Current.State["audiorecordedbytes"] as byte[];


            string url = ServiceOperations.Service_ImageUpload;
            var parameters = new Dictionary<string, object>();
            parameters.Add("FileName", filename);
            parameters.Add("photo", buffer);


            this._currentPostTask = new HttpPostTask(url, parameters, this.OnPostCompleted);
            this._currentPostTask.Execute();
But I want to show Progressbar with percentage, in webclient we are having the progresschangedevent but HttpWebRequest we don't have such type of events.
How can show progressbar with percentage in windows phone8.
Thanks in advance.