Is there a specific need?
You can add JS to any landing page to target form elements if needed.
1. Add HTML block to your landing page
2. Add the script to your landing page template
3. Use a token in your landing page template and set the token value in a program containing the landing page.
I hope it helps
Ryan, pay very close attention to the manual Robb linked to. You must use the Marketo Forms 2.0 event model to enhance Marketo forms. While an experienced JS programmer will have no trouble waiting for events, attempting to skip proper event handling -- or to treat forms like raw HTML forms -- will fail, and you'll lose leads and/or make your forms unusable. There are commercial "data enrichment" form plugins that make these same mistakes with Marketo forms. Respect the framework, though, and you'll be fine!