WebRequest request = (HttpWebRequest)WebRequest.Create(url);
        request.Method = "POST";
        byte[] byteArray = Encoding.UTF8.GetBytes(postData);
        request.ContentType = "application/x-www-form-urlencoded";


        request.PreAuthenticate = true;
        String username = "username";
        String password = "API-Token from Jenkins settings";
        String encoded = System.Convert.ToBase64String(System.Text.Encoding.UTF8.GetBytes(username + ":" + password));
        request.Headers.Add("Authorization", "Basic " + encoded);

        request.ContentLength = byteArray.Length;
        Stream dataStream = request.GetRequestStream();
        dataStream.Write(byteArray, 0, byteArray.Length);
        dataStream.Close();
        WebResponse response = request.GetResponse();


        dataStream = response.GetResponseStream(); //Error appears here (403)
        var reader = new StreamReader(dataStream);
        string responseFromServer = reader.ReadToEnd();
        reader.Close();
        dataStream.Close();
        response.Close();

        return responseFromServer;