Twitter API (401) Unauthorized while attempting to register Twitter webhook

The request below returns The remote server returned an error: (401) Unauthorized and no more information added.

• I can authenticate, send message and get user timeline informations with my tokens.
• I have a callback Uri set on user authentication settings.
• My app has read, write and direct message access permissions.
• I have my dev environment set up for Account Activity API.
• I used different solutions from github but they all return the same error.

Is there anything else I should do before making this request?

            var oauth_token = "xxx-xxxx";
            var oauth_token_secret = "xxxx";
            var oauth_consumer_key = "xxxx";
            var oauth_consumer_secret = "xxxxx";


            // oauth implementation details
            var oauth_version = "1.0";
            var oauth_signature_method = "HMAC-SHA1";

            // unique request details
            var oauth_nonce = new Random().Next(123400, 9999999).ToString(CultureInfo.InvariantCulture);
            var timeSpan = DateTime.UtcNow - new DateTime(1970, 1, 1, 0, 0, 0, 0, DateTimeKind.Utc);
            var oauth_timestamp = Convert.ToInt64(timeSpan.TotalSeconds).ToString(CultureInfo.InvariantCulture);


            // message api details AccActApiTheUSA  AccActApi
            var resource_url = String.Format("https://api.twitter.com/1.1/account_activity/all/MyEnvName/webhooks.json?url={0}", System.Web.HttpUtility.UrlEncode("my callback uri"));

            // create oauth signature
            var baseFormat = "oauth_consumer_key={0}&oauth_nonce={1}&oauth_signature_method={2}" +
                            "&oauth_timestamp={3}&oauth_token={4}&oauth_version={5}";

            var baseString = string.Format(baseFormat,
                                        oauth_consumer_key,
                                        oauth_nonce,
                                        oauth_signature_method,
                                        oauth_timestamp,
                                        oauth_token,
                                        oauth_version
                                        );

            baseString = string.Concat("GET&", Uri.EscapeDataString(resource_url), "&", Uri.EscapeDataString(baseString));

            var compositeKey = string.Concat(Uri.EscapeDataString(oauth_consumer_secret), "&", Uri.EscapeDataString(oauth_token_secret));

            string oauth_signature;
            using (HMACSHA1 hasher = new HMACSHA1(ASCIIEncoding.ASCII.GetBytes(compositeKey)))
            {
                oauth_signature = Convert.ToBase64String(hasher.ComputeHash(ASCIIEncoding.ASCII.GetBytes(baseString)));
            }

            // create the request header
            var headerFormat = "OAuth oauth_nonce=\"{0}\", oauth_signature_method=\"{1}\", " +
                               "oauth_timestamp=\"{2}\", oauth_consumer_key=\"{3}\", " +
                               "oauth_token=\"{4}\", oauth_signature=\"{5}\", " +
                               "oauth_version=\"{6}\"";

            var authHeader = string.Format(headerFormat,
                                    Uri.EscapeDataString(oauth_nonce),
                                    Uri.EscapeDataString(oauth_signature_method),
                                    Uri.EscapeDataString(oauth_timestamp),
                                    Uri.EscapeDataString(oauth_consumer_key),
                                    Uri.EscapeDataString(oauth_token),
                                    Uri.EscapeDataString(oauth_signature),
                                    Uri.EscapeDataString(oauth_version)
                            );


            // make the request

            ServicePointManager.Expect100Continue = false;

            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(resource_url);
            request.Headers.Add("authorization", authHeader);
            request.Method = "GET";
            request.ContentType = "application/x-www-form-urlencoded";
            using (HttpWebResponse response = (HttpWebResponse)request.GetResponse())
            using (System.IO.Stream stream = response.GetResponseStream())
            using (StreamReader reader = new StreamReader(stream))
            {
                return reader.ReadToEnd();
            }