"Sleep" keyboard button/key does not activate S3 suspend

I have a few keyboards from the early 2000s that have a dedicated physical button labeled sleep that is supposed to put the computer into S3 suspend. On the likes of Mint Cinnamon, this indeed occurs but, on Mint Xfce, pressing the button does nothing.

Manually mapping this "Sleep" button via preferences ▶ shortcuts to something and then checking the keyname in ~/.config/Thunar/accels.scm shows that the keyname is simply nothing more than just Sleep