flarum/embed
is to embed Flarum on an external website.
fof/formatting
provides a feature called MediaEmbed which allows embedding external websites into Flarum. There's a whitelist of URLs that will be turned into embeds.
Inserting HTML in posts is essentially not possible. There's a way to whitelist some basic HTML, but iframe
is a dangerous HTML tag to whitelist.
If you need to embed a custom website that isn't supported by MediaEmbed, you can register additional sites in MediaEmbed, or create a custom bbcode for your use case. Mega doesn't seem to be in the list but it can probably be added since it's exactly what MediaEmbed is designed for https://s9etextformatter.readthedocs.io/Plugins/MediaEmbed/Sites/