Hi Kim,
There is two ways you can try to achieve that.
I would go with option 1 first, and if it does not work, give it a go on opti on 2.
Option 1 -
Install ConvertPlus Addon.
Create a modal and insert your code on the custom form field. Test the modal.
If it works, watch this video to learn how to launch from a button.
Option 2 - Use X Theme Lightbox functionality.
On this case you would need to create a page, select the template BLANK NO CONTAINER NO HEADER NO FOOTER
Add your Iframe. test.
If it works, watch the following documentation on how to launch a lightbox.
http://demo.theme.co/integrity-1/shortcodes/responsive-lightbox/
Hope it helps!