If you want to include js, you have a couple of different options. You can use any of the built in events, such as click actions and scene actions to add custom js. If you want to include js on the page generally, you can use Project->Head HTML to paste some inline code or refer to an external script.
As for form elements, I would suggest using your js to insert the form. Alternative, you could edit the exported file, but that would be rewritten every time you export the project.