I just did the same thing - I wanted to embed a SurveyMonkey survey into a Marketo Landing Page. I added the custom HTML but when I do this - my entire footer disappears. Any ideas? Thanks!!
Jessica,
I had no problems after all, with Formbox inserted by iFrame.
The problems about displays on Marketo might be caused by many reasons: versions of web browser, versions of jQuery, etc.
Best,
Tad