How to resolve exception "remote name cannot be resolved" while invoking a request with http URL?

string result = string.Empty;
               //build the request object
               HttpWebRequest request = (HttpWebRequest)WebRequest.Create(txtRequestURL.Text);



               if (chkRequestHeader.Checked)
               {

                   request.Headers.Add("x-api-key", "49b05a5d-36c6-4ed4-98bf-720c2a3414e4");
               }


               request.ContentType = @"application/json";
               request.Accept = @"application/json";
               request.Method = "POST";

               //write the input data (aka post) to a byte array
               byte[] requestBytes = new ASCIIEncoding().GetBytes(txtReq.Text);
               //get the request stream to write the post to
               Stream requestStream = request.GetRequestStream();
               //write the post to the request stream
               requestStream.Write(requestBytes, 0, requestBytes.Length);
               txtLog.Text += "test_http()-process 6 Reached" + Environment.NewLine;
               //build a response object to hold the response
               //submit the request by calling get response
               HttpWebResponse response = (HttpWebResponse)request.GetResponse();
               //get the response stream to read the response from
               Stream responseStream = response.GetResponseStream();
               //now read it out to our result
               using (StreamReader rdr = new StreamReader(responseStream))
               {
                   //set the result to the contents of the stream
                   txtResponse.Text = rdr.ReadToEnd();
               }