1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:

string URL = "https://www.mainlysoft.com/Mainlynet/Administration/Logbuch.php";
                        WebClient webClient = new WebClient();

                        NameValueCollection formData = new NameValueCollection();
                        formData["value1"] = "";
                        formData["value2"] = "";
                        formData["value3"] = "";
                        formData["value4"] = "";

                        byte[] responseBytes = webClient.UploadValues(URL, "POST", formData);
                        string responsefromserver = Encoding.UTF8.GetString(responseBytes);
                        webClient.Dispose();					

1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
13:
14:
15:
16:
17:
18:
19:
20:
21:
22:
23:
24:
25:
26:
27:
28:
29:
30:
31:
32:
33:
34:
35:
36:
37:
38:

System.Net.ServicePointManager.ServerCertificateValidationCallback +=
                    delegate (object sender1, X509Certificate cert, X509Chain chain, SslPolicyErrors sslError) {
                        bool validationResult = true;
                        return validationResult;
                };

                HttpWebRequest myHttpWebRequest = (HttpWebRequest)HttpWebRequest.Create("https://mainlysoft.com/Mainlynet/Administration/test.php");
                // In der unteren Zeile kommt der Fehler
                myHttpWebRequest.Host = "https://www.alfahosting.de";
                myHttpWebRequest.Method = "POST";
                CookieContainer cc = new CookieContainer();
                myHttpWebRequest.CookieContainer = cc;

                myHttpWebRequest.UserAgent = "Mozilla/5.0 (Windows NT 6.1; WOW64; rv:25.0) Gecko/20100101 Firefox/25.0";
                myHttpWebRequest.Accept = "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8";
                myHttpWebRequest.ContentType = "application/x-www-form-urlencoded";
        
                byte[] data = Encoding.ASCII.GetBytes(this.textBox1.Text);
                myHttpWebRequest.ContentLength = data.Length;

                Stream requestStream = myHttpWebRequest.GetRequestStream();
                requestStream.Write(data, 0, data.Length);
                requestStream.Close();

                HttpWebResponse myHttpWebResponse = (HttpWebResponse)myHttpWebRequest.GetResponse();

                Stream responseStream = myHttpWebResponse.GetResponseStream();

                StreamReader myStreamReader = new StreamReader(responseStream, Encoding.Default);

                string pageContent = myStreamReader.ReadToEnd();

                myStreamReader.Close();
                responseStream.Close();

                myHttpWebResponse.Close();

                MessageBox.Show(pageContent);					

1:
2:
3:
4:

NameValueCollection nvc = HttpUtility.ParseQueryString(String.Empty);
nvc.Add("Value1", MeinLieberValue1);
nvc.Add("Value2", MeinLieberValue1);
byte[] data = Encoding.ASCII.GetBytes(nvc.ToString();