GeckoFX

// Change html attribute
 var usernameInput = geckoWebBrowser1.Document.GetElementById("j_username");
  usernameInput.SetAttribute("value", "abc");


// ButtonClick
 var buttonElement = geckoWebBrowser1.Document.GetElementById("button");
 new GeckoButtonElement(buttonElement.DOMElement).Click();


// FormSubmit
 var form = geckoWebBrowser1.Document.GetElementById("loginForm");
  (form as GeckoFormElement).submit();


// Execute JS
 using (AutoJSContext context = new AutoJSContext(geckoWebBrowser1.Window))
  {
     var result1 = context.EvaluateScript(
         "(() => { return 2; })()",
          (nsISupports)geckoWebBrowser1.Window.DomWindow,
         out res
          );

     var result2 = context.EvaluateScript(
         "document.getElementById('loginForm').submit()",
          (nsISupports)geckoWebBrowser1.Window.DomWindow,
         out res
          );
   }