When having an active loop, you can only save the loop to a remix deck slot by dragging the song title.
On the S4MK2 you could click a remix slot button and it would capture the active loop (when source set to the deck you want)
This works on the F1 as well, but not on the S4MK3. DJ Endo is his full tutorial also says, you can also click the pad, but it doesn't work and then he say's, we'll just drag the song title.
Is there a mapping to make it work?