string loginURI = "http://centreon.company.by/";
        string username = "login";
        string password = "pass";
        string reqString = "useralias="+username+"&"+password +"=pass&submit=Connect+%3E%3E";
        byte[] requestData = Encoding.UTF8.GetBytes(reqString);

        var request = (HttpWebRequest)WebRequest.Create(loginURI);
        request.AllowAutoRedirect = true;
        request.Method = "POST";

        request.ContentType = "application/x-www-form-urlencoded";
        request.ContentLength = requestData.Length;

        using (Stream S = request.GetRequestStream())
            S.Write(requestData, 0, requestData.Length);

        using (var response = (HttpWebResponse)request.GetResponse())
        {
            var newPageCode = new StreamReader(response.GetResponseStream()).ReadToEnd();
        }