C#.net docusign api call return error ( the underlying connection was closed: an unexpected error occurred on a receive)

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C#

API

C#.net DocuSign Api call return error ( The underlying connection was closed: An unexpected error occurred on a receive).DocuSign URL contain two part (baseURL,ResourcePath). sending document for sign as base64string.see the following code.

What I have tried:

private void Form1_Load(object sender, EventArgs e)
        {           
            String baseURL = "https://prime-dsa-devctr.docusign.net:8081";
            String resourcePath = "sapiws/v1/digital_signature";
            String apiUrl = baseURL + "/" + resourcePath;
            var client = new RestClient(apiUrl);            
            var request = new RestRequest(Method.PUT);
            request.AddHeader("content-type", "application/json");
            request.AddHeader("authorization", "Basic " + DSABasicAuthorizationString("sudeep@cse.com.sa", "####"));
            // "digital_signature" request body as Json formated String (use JavaScriptSerializer or Newtonsoft.Json to build from object) 
            String DigSigRequestBody =
                "{ \"CreateAndSignField\" : " + //structure name specifies the operation / function
                    "{   \"file\": " + "\"" + File2Base64String("C:\\work\\PurchaseOrder.pdf") + "\", " +
                        "\"fileType\": \"PDF\", " +
                        "\"x\": \"91\", " +
                        "\"y\": \"164\", " +
                        "\"width\": \"113\", " +
                        "\"height\": \"38\", " +
                        "\"page\": \"1\", " +
                        "\"timeFormat\": \"h:mm:ss\", " +
                        "\"dateFormat\": \"dd/MM/yyyy\", " +
                        "\"appearance\": [\"GRAPHICAL_IMAGE\", \"SIGNED_BY\", \"TIME\"]" +
                    "}" +
                "}";
            request.AddParameter("application/json", DigSigRequestBody, ParameterType.RequestBody);            
            IRestResponse response = client.Execute(request);            
            JObject joResponse = JObject.Parse(response.Content);

            Base64String2Path(joResponse["signedFile"].Value<string>(), "C:\\work\\PurchaseOrder.DSA-REST-SIGNED.pdf");
            Console.WriteLine("success");
            Console.ReadKey();
        }

private static void Base64String2Path(string Base64String, string FilePath)
        {
            Byte[] bytes = Convert.FromBase64String(Base64String);
            File.WriteAllBytes(FilePath, bytes);
        }

        private static String File2Base64String(String FilePath)
        {
            Byte[] bytes = File.ReadAllBytes(FilePath);
            String fileB64Data = System.Convert.ToBase64String(bytes);
            return fileB64Data;
        }

        private static String DSABasicAuthorizationString(String username, string password)
        {
            var DSABasicAuthorizationBytes = System.Text.Encoding.UTF8.GetBytes(username + ":" + password);
            return System.Convert.ToBase64String(DSABasicAuthorizationBytes);
        }