How to "reply as topic"?

[int19h (Pavel Minaev)]

I keep seeing references to the ability to reply to a topic by creating another topic, but I can’t find the UI that would actually let me do that. It looks like it was a menu option when clicking on the dots at some point, but now that it brings up a bunch of icons, I don’t see reply as topic there - only things like “like”, “flag”, and “delete”.