You can do this with a custom template, no need for iframe really.
Take a look here - http://doc.silverstripe.com/doku.php?id=form#using_a_custom_template
Basically you'll have to write your own form and not build one in the admin CMS area.
If you're looking at having the form show up in the admin area, not sure that is possible with a user defined form/custom template, though as I'm writing this I think I might have read something in the forums on that once. Maybe a developer can shed more light.