bibekshakya12 what is your use case?
There is no extension allowing arbitrary iframe insertion at this time. It would be a very dangerous thing to give access users to.
However the MediaEmbed feature which can be enabled with https://discuss.flarum.org/d/17770-friendsofflarum-formatting transforms links to known and safe services into embedded iframes. It's also possible to define additional rules if you wanted to integrate one of your own services.