We are running into this same issue. Has anyone successfully been able to submit hidden fields by embedding a second form into the Custom HTML?
You don't need to add a second form. There already is an otherwise functioning Marketo form with which you can use addHiddenFields. I'll put this on my blog tonight and post a link.
Hi Sanford - would you mind sharing your blog post on adding the hiddenfields to the custom HTML form? Thanks!