ART994 Please read Clark's post, it answered everything you've asked about.
The code you posted is HTML code, which cannot be used in a Flarum post (to render anything, including iframes). It will not work.
The options you have are to create a custom extension, or use FoF Formatting and extend the MediaEmbed function (put simply, this is what lets Formatting embed videos like YouTube in a post) with extend.php
. This will allow you to post a link from protonvideo.to and have it embed in a post.
If you're comfortable doing this, you're free to ask questions to figure out how to accomplish it but please read the manual that Clark linked first, as well as our Getting Started guide in the docs.
If you're not comfortable doing this, feel free to open a new discussion in the Resources > Services tag to commission an extension. Most community developers who take commissioned work do so for payment, please be aware of that. You can also solicit developers from outside the Flarum community, there's any number of such services on the internet where you can do so.
Hope that helps.