In the web UI, it is the button that looks like two rectangles (copying one of them I suppose), between the star icon (favorite) and the three vertical eclipses (moar stuff), second from the right.

But if you use an app, ymmv.

I hope you can cross-post bc this way breaks several things - clicking it takes you away from your instance, it shows up as a duplicate post, etc. - but anyway thank you for sharing this cool thing you saw on the internet (I didn't see the original so I may have missed it if you hadn't)!:-)

If you see something, share something 😁.