Filling a form in an external webpage

Hi all,
I was having a windows application project where I was using webbrowser control to load an external webpage and simulate user to fill in a form in this external webpage. Now I want to do the same thing using asp.net but as you know there is no webbroser control in there. I have loaded the external webpage using AgilityPack but I couldn't edit the textbox in the external webpage. Here is the code that I am using:

String Url2dehands = "http://www.hereisthewebsite.be"; HtmlWeb mvWeb = new HtmlWeb(); HtmlDocument doc = mvWeb.Load(Url2dehands); HtmlNode mvTextBox = doc.DocumentNode.SelectSingleNode("//input[@id='id_ofthe_textbox']"); mvTextBox.InnerHtml= "assdsda"; This won't change the textbox text. I tried other options as well but no joy. Could you please tell me how I can do it?
Many thanks.
...