I saw that a similar question to this was asked and answered in 2011. I need to make one of my forms secure, as people are entering third party credentials into it. The resolution to this question was that Marketo can only make ALL pages secure and that it was a multi-week project that was undergoing streamlining.
So is there a more current method of securing a landing page and form?
If you're emebedding a form with your embed code on an external page which is already secured with https, then the form will submit over https. However, if you're using a marketo landing page for the form fillout you will need to work with our services department(email@example.com) to secure the subdomain which you use for your Marketo landing pages.