GetAccessToken.cs

private static void GetAccessToken()
        {
            try
            {
                SHAREPOINT_TOKEN = string.Empty;
                WebRequest request = WebRequest.Create("https://accounts.accesscontrol.windows.net/" + TENANT_ID + "/tokens/OAuth/2");
                request.Method = "POST";
                string postData = "grant_type=client_credentials" +
                "&client_id=" + WebUtility.UrlEncode(CLIENT_ID + "@" + TENANT_ID) +
                "&client_secret=" + WebUtility.UrlEncode(CLIENT_SECRET) +
                "&resource=" + WebUtility.UrlEncode(SHAREPOINT_RESOURCE_ID + "/" + SHAREPOINT_DOMAIN + "@" + TENANT_ID);

                _logger.LogInformation($"postData: {postData} ");

                _logger.LogInformation("GetBytes: postData ");
                byte[] byteArray = Encoding.UTF8.GetBytes(postData);
                request.ContentType = "application/x-www-form-urlencoded";
                request.ContentLength = byteArray.Length;

                _logger.LogInformation("GetRequestStream: request ");
                Stream dataStream = request.GetRequestStream();
                dataStream.Write(byteArray, 0, byteArray.Length);
                dataStream.Close();

                _logger.LogInformation("Calling: request.GetResponse ");
                using WebResponse response = request.GetResponse();

                _logger.LogInformation("GetResponseStream: response ");
                dataStream = response.GetResponseStream();
                StreamReader reader = new StreamReader(dataStream);
                string responseFromServer = reader.ReadToEnd();
                reader.Close();
                dataStream.Close();

                _logger.LogInformation($"responseFromServer: {responseFromServer} ");

                const string accessToken = "access_token\":\"";
                int clientIndex = responseFromServer.IndexOf(accessToken, StringComparison.Ordinal);
                int accessTokenIndex = clientIndex + accessToken.Length;
                SHAREPOINT_TOKEN = responseFromServer.Substring(accessTokenIndex, (responseFromServer.Length - accessTokenIndex - 2));
            }
            catch (Exception ex)
            {
                _logger.LogError(ex.Message);
                throw;
            }
        }