The embed code provided should work just fine. Where in your html are you adding the code? Need some more context to know for sure. The embed code for forms is short because you are just referencing a form id in your instance, nothing in the embed code itself will show fields.
<script>MktoForms2.loadForm("//app-xxx.marketo.com", "000-xxx-000", 1000);</script>
Nina, nearly all the forms recipes I've posted on the Community use the embed code on my CodePen site. So those are hundreds of working examples in the wild (and of course there are tens, maybe hundreds of thousands more out there).
Hey Nina, you should be able to use the Embed code without any issue, as long as you're posting the entire thing onto your page.
As far as the API goes, the ONLY time you need to worry about the API is if you don't want to use a Marketo Form for whatever reason, you don't need to use the api when you're using a "native" marketo form even on a non-marketo landing page.
Is it possible that something on the LP is conflicting with the form? IE there's nothing wrong with the embed code but something on the LP is not playing nice with it.