One way to do this is to go into Site Setup -> HTML Filtering, remove iframe as a nasty tag and as a stripped tag, then create a page and insert your
Another way is to install the WindowZ product. Installation (as of 2011-01-27) is via the traditional route of downloading the tarball and decompressing into the /products folder of the instance. Once installed into the site via portal_quickinstaller, do this:
Navigate to the location where you wish to embed the form from an outside site and click “Add New”
Give the Window page a title (it will be hidden by default)
Paste in the url of the external form
Change the Width and Height to avoid scroll bars. Good widths that work pretty well range from 600-1000px. Good heights start at 700px and go up. You can come back to change these values. With the Mimbo theme the standard main content area is 600px, that allows the portlets to to be shown.
If you want to see the title and the other metadata unclick the box next to “Hide Metadata”
You may also have to hide the portlets for this page if your form is too wide. Tack on /@@manage-portlets to the URL of this content item.