Access Html Codes With C#

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C#

HTML

C#4.0

Hi,I want access reg.pnu.ac.ir/Forms/AuthenticateUser/main.htm html code that open in
Internet Explorer and access input textBox in my c# app.

when I choose view page source in internet explorer i see input textBox tag and tag's Id
but when I use htmlElementCollection Or htmlElement in my code , I can't see <input> tag
to set attribute it.

what i do ?

I write this code but didnt work.

foreach (HtmlElement el in webBrowser1.Document.All)
   {
       switch (el.Id)
       {
           case "F80351": el.InnerText = "880139701"; break;
           case "F80401": el.InnerText = "3342201"; break;
           case "btnLog": el.InvokeMember("click"); break;
       }
   }