We don't run the javascript tags included in the content. It's just inserted using Mithril's m.trust, which uses innerHTML, which doesn't run JS tags.
I think you can call custom code from an onload handler in the HTML however. So you can load your script in the browser using footer code, then call it from there.
Or if you're trying to build an actual feature, it's best to use Flarum extension API directly and create your own Flarum page outside of this extension.
