Auto login by using HttpWebRequest - httpwebrequest

Question

I'm writing a window application by C# to auto login & post topic to a website. I've tried to pass username & password as the method following but the responseText did not changed. I meant if I passed the incorrect username or password the responseText would be included the "Error text such: wrong password or username" or if I passed to right username & password the responseText would be included "Login successfully" but the responseText I received was like a original login page
Do you have any idea? Thank you so much
private string Login()
{
HttpWebRequest request;
CookieContainer cookieContainer = new CookieContainer();
string cookieHeader = string.Empty;
byte[] byteArray = Encoding.UTF8.GetBytes("username=abc#gmail.com&password=123");
request = (HttpWebRequest)WebRequest.Create("http://www.vietnamworks.com/dang-nhap");
request.ContentType = "application/x-www-form-urlencoded";
request.UserAgent = userAgent;
request.Method = "POST";
request.CookieContainer = cookieContainer;
request.ContentLength = byteArray.Length;
using (Stream os = request.GetRequestStream())
{
os.Write(byteArray, 0, byteArray.Length);
}
HttpWebResponse response = (HttpWebResponse)request.GetResponse();
string pageSource;
using (StreamReader sr = new StreamReader(response.GetResponseStream()))
{
pageSource = sr.ReadToEnd();
}
return pageSource;
}