public const string userAgent = "Mozilla/5.0 (Windows NT 6.1; rv:7.0.1) Gecko/20100101 Firefox/7.0.1";
        public CookieContainer cookieJar;

        private CookieContainer Login(string username, string password, string loginPageURL)
        {
            // Create a request using the provied URL. 
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(loginPageURL);

            // Set the Method property of the request to POST.
            request.Method = "POST";

            // Set the Cookiecontainer
            CookieContainer cookieJar = new CookieContainer();
            request.CookieContainer = cookieJar;

            // Create POST data and convert it to a byte array.
            //string postData = "username=" + username + "&password=" + password;
            //byte[] byteArray = Encoding.UTF8.GetBytes(postData);

            // Set the ContentType property of the WebRequest.
            request.ContentType = "application/x-www-form-urlencoded";

            // Set the User Agent
            request.UserAgent = userAgent;

            // Set the ContentLength property of the WebRequest.
            //request.ContentLength = byteArray.Length;

            // Get the request stream.
            Stream dataStream = request.GetRequestStream();

            // Write the data to the request stream.
            //dataStream.Write(byteArray, 0, byteArray.Length);

            // Close the Stream object.
            dataStream.Close();

            // Get the response.
            HttpWebResponse response = (HttpWebResponse)request.GetResponse();

            // Get the stream containing content returned by the server.
            dataStream = response.GetResponseStream();

            // Open the stream using a StreamReader for easy access.
            StreamReader reader = new StreamReader(dataStream);

            // Read the content.
            string responseFromServer = reader.ReadToEnd();

            string cookie = cookieJar.GetCookieHeader(request.RequestUri);

            Login ID = JsonConvert.DeserializeObject<Login>(responseFromServer);

            this.PlantID.Value = ID.Back.Data[0].PlantId;

            // Clean up the streams.
            reader.Close();
            dataStream.Close();
            response.Close();

            return cookieJar;
        }

        private string ViewPageGET(CookieContainer cookieJar, string pageURL)
        {
            string str = string.Empty;

            HttpWebRequest request2 = (HttpWebRequest)WebRequest.Create(pageURL);

            // Set the Method property of the request to GET.
            request2.Method = "GET";

            // Put session back into CookieContainer
            request2.CookieContainer = cookieJar;

            // Get the request stream.
            Stream dataStream2 = request2.GetRequestStream();

            // Close the Stream object.
            dataStream2.Close();

            // Get the response.
            HttpWebResponse response2 = (HttpWebResponse)request2.GetResponse();

            // Get the stream containing content returned by the server.
            dataStream2 = response2.GetResponseStream();

            // Open the stream using a StreamReader for easy access.
            StreamReader reader2 = new StreamReader(dataStream2);

            // Read the content.
            str = reader2.ReadToEnd();

            return str;
        }

        public override void Execute()
        {
            if (this.Trigger.WasSet && this.Trigger.HasValue)
            {

                 try
                {
                    // URL für Login
                    string loginPageURL = "https://server.growatt.com/newTwoLoginAPI.do?userName=" + this.Benutzername + "&password=" + MD5Passwort;

                    // URL für Jahresproduktion - GET
                    string pageURL2 = "https://server.growatt.com/newPlantDetailAPI.do?plantId=" + this.PlantID + "&date=" + DateTime.Now.Year.ToString() + "&type=3";

                    // URL für Tagesproduktion - GET
                    string pageURL3 = "https://server.growatt.com/newPlantDetailAPI.do?plantId=" + this.PlantID + "&date=" + DateTime.Now.Date.ToString("yyyy-mm-dd") + "&type=1";

                    // Login-Aufruf
                    cookieJar = Login(this.Benutzername, this.Passwort, loginPageURL);

                    // Aufruf 2. API (GET)
                    Root2 Growattlogin2 = JsonConvert.DeserializeObject<Root2>(ViewPageGET(cookieJar, pageURL2));

                    // Aufruf 3. API (GET)
                   Root2 Growattlogin3 = JsonConvert.DeserializeObject<Root2>(ViewPageGET(cookieJar, pageURL3));

                    this.Year.Value = (float) Convert.ToDouble(Growattlogin2.Back.PlantData.CurrentEnergy.Replace(" kWh", ""));
                    this.Today.Value = (float) Convert.ToDouble(Growattlogin3.Back.PlantData.CurrentEnergy.Replace(" kWh", ""));

                    this.Fehler.Value = string.Empty;
                }
                catch (Exception ex)
                {
                    this.Fehler.Value = ex.Message;
                }
            }
}

WebRequest request = WebRequest.Create(uri);
request.Method = "GET";
request.CookieContainer ....
using (WebResponse response = request.GetResponse())
using (Stream stream = response.GetResponseStream())
{
      ...
}

        private string ViewPageGET(CookieContainer cookieJar, string pageURL)
        {
            string str = string.Empty;

            WebRequest request = WebRequest.Create(pageURL);
            request.Method = "GET";
            request.CookieContainer = cookieJar;
            using (WebResponse response = request.GetResponse())
            using (Stream stream = response.GetResponseStream())
            {
                StreamReader reader = new StreamReader(stream, Encoding.UTF8);
                String responseString = reader.ReadToEnd();
                return responseString;
            }
        }