dotails Hi and welcome to Flarum!
The Insert Code button on our post composer is meant for showing actual code, which is useful on forums because there are many times where sharing code or markup is hard to do in the formatted text of a post. It is not, however, meant for rendering code. So if you place an iframe, or any other kind of code within a block generated by Insert Code, it will not render and show the contents of another website within an iframe.
To do something like that, you would need an extension or to consider another approach. If you want to keep discussing this, please open a new discussion in the Support tag and explain what you're hoping to do with an iframe, and we can give you more guidance on achieving it.
The code formatting via BBCode and Markdown was not broken in Beta 12 and has no changes applied in Beta 13. So it remains the same method as I described above.
