private void button4_Click(object sender, EventArgs e)
{
    WebBrowser browser = webBrowser1;
    var textboxes = browser.Document.GetElementsByTagName("input");
    foreach (HtmlElement textbox in textboxes)
    {
        if (textbox.GetAttribute("name") == "add_comment_text")
        {
            textbox.SetAttribute("value", "newValue");
        }
    }
}