A link preview BBcode: [preview=].
A Flarum extension which adds a new BBCode called preview. This allows you to post a link with a maginifying glass icon to the right of the link. Hover over or tap on the magnifying glass icon and a preview will load via an iframe. Some websites will not load in iframes; they don't allow it. If you run into this, the preview simply will not load and you will see a browser error. This is your signal to edit your post and remove the preview BBCode. Better luck next time. ;)
Here is the BBCode (preview equals the URL):
composer require zerosonesfun/flarum-ext-link-previews
composer update zerosonesfun/flarum-ext-link-previews php flarum cache:clear
Also, after updating, deactivate and then reactivate the extension. Clear your browser cache too.