How do I embed one topic inside another?

I feel dumb asking, but I couldn’t find the answer in seach. I see users embedding one topic inside another and it gets a nice preview box that can be expanded to the full viewl. However, this doesn’t work when I try it on my own site. Is this a plugin or is there a special format for dropping the link in.

Apologies if this has be answered and I missed it.

Simply paste the link on a line by itself. This is called “oneboxing” and is covered in the welcome PM that every new user receives.

Details here: