Great question, fortunately I have refrained from using a sub until now. It seems to be complicated!
According to the manual (
http://manuals.denon.com/AVRX3500H... for 5.1, 7.1 also has only one sub, therefore the same setup), you have to connect the subwoofer to the pre-out you described. The same connection is also found on active subs and seems to be a cinch? Pre-out means signal without amplification (in contrast to the banana plug connections at the bottom for the other speakers).
Regarding cables, I only found the following:
Inakustik Premium Y-Subwoofer Cable (3m, Premium, Cinch) A Y-cable with two cinch on one side. The sub has as many inputs as the AV outputs.
According to the following source (
https://www.heimkino-praxis.com/subwoofer-...), Y-cables are common for subs because the latter have the typical two inputs red and white. If you only have one (=mono), I would simply plug in one connector of the Y-end, as this does not seem to make any significant difference according to the source.