Can't blame the devs, the fact that in the docs the Settings and Skins sections look similar is absolute shit and confused me for the longest time as well.
That being said, DesktopWorkArea goes in the global setting file, rainmeter.ini, which you can access by clicking that Edit settings button (that's next to Refresh all)
You can modify it using lua (or better yet, do some stuff in it using PowershellRM or any other good terminal) but the devs are gonna scold me so, go your own way :D