There's not really such a thing in Flarum.
There's a special "reply" key that can be used in the URL but it's intended for internal use and could stop working at any moment in future Flarum versions, like https://discuss.flarum.org/d/4154-friendsofflarum-upload-the-intelligent-file-attachment-extension/reply Using that probably also causes suboptimal performance because Flarum cannot preload the correct part of the discussion and ends up downloading the posts twice. Also it might not always work when logged out.
When browsing inside of the Flarum UI, links dynamically get the last post you have read to generate a links in the discussion list. If you are listing discussions and links yourself, you can obtain the last post ID from the discussion object and use that in your link.
The safest solution would be to implement that yourself using a Flarum extension.
