Can you please be precise what forum you are talking about? Some forums don’t allow them so you need to check their rules before registering. Else you may use basic html tags in posts but that again is limited.
It is all part of of bypassing the content filters. It is dangerous, but I wrote a plugin once for allowing embeds by the administrator and _ck_ wrote a better one!.
https://bbpress.org/plugins/topic/admin-can-post-anything/
Not recommended for an open forum though since it would just be the biggest security risk ever to have it open to everyone….
Trent