Docusign无法登录。如何在C#代码中登录？

using System;
using System.Net;
using System.IO;
using System.Text;
using Newtonsoft.Json.Linq;

HttpWebRequest request = (HttpWebRequest)WebRequest.Create("https://account-d.docusign.com/oauth/token");
request.Method = "POST";
request.ContentType = "application/x-www-form-urlencoded";

string clientId = "Your_Client_Id";
string clientSecret = "Your_Client_Secret";
string username = "Your_Docusign_Username";
string password = "Your_Docusign_Password";
string integratorKey = "Your_Integrator_Key";

string postData = $"grant_type=password&client_id={clientId}&client_secret={clientSecret}&username={username}&password={password}&scope=signature%20impersonation&assertion_type=urn:ietf:params:oauth:grant-type:jwt-bearer&assertion={integratorKey}";
byte[] byteArray = Encoding.UTF8.GetBytes(postData);
request.ContentLength = byteArray.Length;

Stream dataStream = request.GetRequestStream();
dataStream.Write(byteArray, 0, byteArray.Length);
dataStream.Close();

HttpWebResponse response = (HttpWebResponse)request.GetResponse();
string responseString = new StreamReader(response.GetResponseStream()).ReadToEnd();

JObject jsonResponse = JObject.Parse(responseString);
string accessToken = jsonResponse["access_token"].ToString();