string textbox = textbox.Text //Replace "textbox" with your textbox name

try
{
  WebClient WC = new WebClient();
  string tb_contents = WC.DownloadString(textbox);

  WC.DownloadString(textbox);
  if (!tb_contents.Contains("html>")) //If link contains raw text
  {
    api.SendLimitedLuaScript(tb_contents); //Executes text in the link
  }
  else //If link doesn't contain raw text
    MessageBox.Show("The link didn't raw text.", "title");
}
catch //If link is invalid
{
  Messagebox.Show("The text wasn't a valid link.", "title");
}