If you link to a topic or post, it becomes embedded in a box, like this:
To link to a topic: find the topic in the list of topics and click on it to open it. Then, either copy the link from your browser’s address bar or click on the small “Share” icon on the top right of the very first post.
To link to a post: do as above but click on the “Share” button at the top right of the post itself.
When you’ve added the link, a black bar will appear at the bottom of the editor, saying: “Your link has been automatically embedded. Display as a link instead.” If you want to keep it embedded, you can close this black bar and the “X” button to close the black bar will tell you “Keep embedded content”. It will automatically close itself if you start typing.
Note: It's best to hit the Enter key to make it work, as it needs to be on a separate line by itself and not be inline with text you've written. It may take a few seconds for the embedded link to appear, so you may need to wait a little while. If you click "Submit" before the link is fully embedded, it won't be embedded and will show as a normal link.