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It can be done, but it's a mod. It would be a security issue to allow anyone to add an iframe on your site. So, it would need to be limited to admins. Because of that, it requires a mod... which I don't want to write.

You'll need to look at parse_bbc() in Subs.php and get a pretty good idea of how it all works.

May be an addon to enable only iframe attribute in HTML.

@Joshua Dickerson I know a little bit of php (still a newbie) so if you could point me to the right direction, I could probably  take a shot. thanks

You need to either do it in preparsecode() or check it in there and do it in parse_bbc(). It's not a simple mod because it requires permissions to do it. If you are new to PHP/Elkarte, I doubt you have the skills or knowledge to do it right now without a lot of help. Sorry, I don't have time to do this.

First few random questions: any user have to be able to do that? Do you need it on "several" topics, or it's just a "one-topic" thing to explain how to use the calculator?
In one case (i.e. multiple users and/or multiple times) it would be worth write an addon for the specific topic. But if you need it just one time (or very few times) to explain how to use the form, then use the BBC html addon you can find at http://addons.elkarte.net/