To edit the HTML in the form editor you have to click the </> icon. This lets you enter or paste HTML code into the editor.
to have the modal window display an iframe you'd have to enter something similar to:
/<`iframe src="http://YourSite.com/PageURL/" frameborder="0" height="500px" width="100%" scrolling="yes" align="center">/
*you'll need to put your URL of the page you are wanting in the iframe in "'s and you'll need to remove the leading and following /
Then hit the </> button and save.