Need help: Energy ESW-C10 subwoofer -- Denon Avr-s540bt receiver?

Amazon.com: KabelDirekt – 10ft long – RCA/phono Y cable, 1 to 2 RCA/phono, stereo audio cable (coax cable, RCA/phono male/male plugs, analog/digital, adapter for subs/amps/Hi-Fis/home theater/receivers, black) : Electronics

This will work for your Sub.  I once had an older sub for this type cable with Y on opposite end

You could also use any length of sub/rca cable and use a short Y splitter with 1 female in to 2 male outs.
I have an old Velodyne sub like that.  It said you could run a sub out to either right or left input or use a Y connecter.

Thank you both.