how to download page source when some operation on page is needed

I am using below code to download my target web site source and extract my required information .

    protected void Button1_Click(object sender, EventArgs e)
    {
        WebClient Client = new WebClient ();
        Client.DownloadFile("target page url", " D:/myTextFile.txt");
    }

But in some cases in my target web site i need to press a key to load my required information. For example in this page I should press "More" Button to see full data. How can i reach this object?