WebClient client = new WebClient();
client.UseDefaultCredentials = true;
client.Proxy.Credentials = CredentialCache.DefaultCredentials;
client.Encoding = Encoding.UTF8;
String s = client.DownloadString(textBox1.Text);
The line:
client.Proxy.Credentials = CredentialCache.DefaultCredentials;
is needed if you have a proxy. This uses the default setting from Windows.