Sounds like you'd need the Form to use Known Lead HTML. But you may need it to be fancier because you want the first group of people to get a form, not a Button.
It's possible RTP can do this, however, is that really worth it to you?
Embedded Marketo forms, the longer one w/KL HTML, like Josh said. You don't need anything iframed.
Note: there's no such thing as a Marketo form in an iframe vs. Marketo LP in an iframe. You always need an LP, even if it does not contain anything other than barebones markup.
Thanks Josh and Sanford!
Also, just interested in confirming it's truly the case that:
iframe LP = can do pre-fill functionality but not 'known/unknown lead' / 'hide form/show form' functionality
Form embed = can do 'known/unknown lead' / 'hide form/show form' functionality but not pre-fill functionality
No, that's not the case. All forms support KL HTML.
PreFill is not supported natively for embeds$
If you only want to fill in the Email Address, though, you don't need PreFill proper. You can store the entered address in a cookie and also append the email address to your tokenized links. This will cover the same bases.