private void Form1_Load(object sender, EventArgs e)
{
  webBrowser.Navigate("file:///C:/Users/Standardbenutzer/Desktop/Win8_HTML/win.html");
  webBrowser.Navigated += webBrowser_Navigated;
}

void webBrowser_Navigated(object sender, WebBrowserNavigatedEventArgs e)
{
  InjectScript();
}

private void InjectScript()
{
  string script =
    @"function setScript()
    {alert('injected');}";
  appendScriptElement(script);
  webBrowser.Document. InvokeScript("setScript");
}

private void appendScriptElement(string script)
(
  var headElements webBrowser.Document.GetElementsByTagName("head");
  var headElement headElements[0];
  var scr webBrowser.Document.CreateElement("script");
  scr.SetAttribute("text", script);
  headElement, AppendChild(scr);