I have the error "System.Net.WebException: 'The remote server returned an error: (403) Forbidden." while connecting to a website in c#

I'm connecting to a website with HttpWebRequest but I have the error System.Net.WebException: 'The remote server returned an error: (403) Forbidden.'

Here is my code :

var request = (HttpWebRequest)WebRequest.Create(newUri);
        request.Method = "POST";
        request.CookieContainer = new CookieContainer();
        request.ContentType = "application/x-www-form-urlencoded";
        byte[] bytes = Encoding.ASCII.GetBytes("username=" + entryLogin + "&password=" + entryPassword + "&selection=" + SELECTION + "&codeFournisseurIdentite=" + CODE_FOURNISSEUR_IDENTITE + "&execution=" + executionKey + "&_eventId=" + EVENT_ID + "&geolocation=&submit=" + SUBMIT_ACTION);
        request.ContentLength = bytes.Length;
        request.Credentials = CredentialCache.DefaultCredentials;
        request.MaximumAutomaticRedirections = 3;
        request.AllowAutoRedirect = true;

        await Task.Delay(30000);

        using (HttpWebResponse webSiteresponse = (HttpWebResponse)request.GetResponse())
        {
            foreach (Cookie cook in webSiteresponse.Cookies)
            {
                cookieContainer.Add(cook);
            }
        }

I didn't understand what is wrong... Can someone help me ?