My method is preferred because you don't have to load/hide a bunch of stuff you don't need for the form. It simply loads the form and the required scripts/css to make it work.
You would take that code snippet and save it as a new file in the root of your theme folder. You can call it plainpage.php or plain_page_template.php or whatever you'd like.
Once you've done that, you want to create a new page in the WordPress admin. This page is going to be the one to host your modal form. Embed your form in the content body the normal way. Then, look in the right sidebar for the "template" selector. Click on that and select your new plain page template from the list. Then, you just save the page as usual.
You can then click on the "view page" link at the top of the page. You should jump to the new page with just the form and no other content. Copy the URL and use that as the URL for your iframe embed.
I went into some detail on the original post here. You can refer back to that for more info.
Also, here's some more information on creating WordPress page templates
Posted 10 years ago on Wednesday August 18, 2010 | Permalink