ash3T I just updated it, or, upgraded it. I used a new method to achieve it. I'm not quite sure if it would work on all flarum websites, but I think it worth a try.
You can update your extension to v2.0.0, and this version make you able to directly use <iframe> element and some of its attributes. For example, you can use
<iframe src="URL"></iframe>
Also, attributes like "style", for inline css are allowed, thus, you can use any of the following attributes, just the same as writing in HTML:
<iframe src="URL" style="CSS" scrolling="yes/no" allow="attribute"></iframe>
If you need help in how to use <iframe> element, you can read this MDN doc : https://developer.mozilla.org/en-US/docs/Web/HTML/Element/iframe .
I'm planning to add all iframe attributes available in this extension in the future.
Wish you all the best.
p.s. If you have any feedback, good, or bad, please tell me.