Advanced iFrames
By default, <iframe> is blocked for security reasons. However, if the iframes are from highly trusted sources, a Discourse admin can whitelist that domain as an iframe target with the allowed iframes site settings.
You may also need to paste the full embed code, such as the one for the map below:
<iframe src="https://www.google.com/maps/embed?pb=!1m18!1m12!1m3!1d24202.451099423397!2d-74.06201522657554!3d40.68924937923285!2m3!1f0!2f0!3f0!3m2!1i1024!2i768!4f13.1!3m3!1m2!1s0x89c25090129c363d%3A0x40c6a5770d25022b!2sStatue+of+Liberty+National+Monument!5e0!3m2!1sen!2suk!4v1533203877892" width="600" height="450" frameborder="0" style="border:0" allowfullscreen></iframe>
This results in: