Смотрим видео на Youtube (что-то вроде накрутчика просмотров)

[DllImport("user32.dll", CharSet = CharSet.Auto, CallingConvention = CallingConvention.StdCall)]
        public static extern void mouse_event(int dwFlags, int dx, int dy, int cButtons, int dwExtraInfo);
 
        private const int MOUSEEVENTF_LEFTDOWN = 0x02;
        private const int MOUSEEVENTF_LEFTUP = 0x04;
        private void Click(int x ,int y)
        {
           // x и y координаты кнопки Play
          mouse_event(MOUSEEVENTF_LEFTDOWN | MOUSEEVENTF_LEFTUP, x, y, 0, 0);
        }

SendKeys.Send();

HtmlElement head = webBrowser1.Document.GetElementsByTagName("head")[0];
HtmlElement scriptEl = webBrowser1.Document.CreateElement("script");
IHTMLScriptElement element = (IHTMLScriptElement)scriptEl.DomElement;
element.text = "function playVideo() { yt.player.playerReferences_.player1.api.playVideo(); }";
webBrowser1.Document.InvokeScript("playVideo()"); //не помню синтаксис, вызывается так или playVideo

HtmlElement head = webBrowser1.Document.GetElementsByTagName("head")[0];
HtmlElement scriptEl = webBrowser1.Document.CreateElement("script");
IHTMLScriptElement element = (IHTMLScriptElement)scriptEl.DomElement;
element.text = "function playVideo() { yt.player.playerReferences_.player1.api.playVideo(); }";
head.AppendChild(scriptEl);
webBrowser1.Document.InvokeScript("playVideo()");