public static bool ValidateCertificate(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
{
    //server sertifikası direkt olarak onaylanıyor
    return true;
}
public string getAccessToken(string url, string userName, string password, string firmNr)
{
    try
    {
        HttpWebRequest req = WebRequest.Create(new Uri(url)) as HttpWebRequest;
        req.Method = "POST";
        req.ContentType = "application/json";
        req.Accept = "application/json";
        req.Headers.Add("Authorization", "Basic " 
               + Convert.ToBase64String(System.Text.Encoding.UTF8.GetBytes(clientId + ":" + clientSecret)));
        req.Credentials = CredentialCache.DefaultCredentials;
        
        //sertifikanın validate edildiği fonksiyonumuza yönlendiriyoruz
        ServicePointManager.ServerCertificateValidationCallback 
               += new System.Net.Security.RemoteCertificateValidationCallback(ValidateCertificate);
        
        byte[] formData = UTF8Encoding.UTF8.GetBytes("grant_type=password"
            + "&username=" + userName
            + "&firmno=" + firmNr
            + "&password=" + password);
        req.ContentLength = formData.Length;
        using (Stream post = req.GetRequestStream())
        {
            post.Write(formData, 0, formData.Length);
        }
        string result = null;
        using (HttpWebResponse resp = req.GetResponse() as HttpWebResponse)
        {
            StreamReader reader = new StreamReader(resp.GetResponseStream());
            result = reader.ReadToEnd();
        }
        dynamic j = JsonConvert.DeserializeObject(result);
        accessToken = j.access_token;
    }
    catch (Exception){}
    return accessToken;
}