How do I take a screenshot in SteamOS’s Desktop Mode?

I want to take a screenshot of my Steam Deck’s desktop in SteamOS 3.3.1. Normally I can take a screenshot by pressing Steam + R1, but this shortcut doesn’t work in Desktop Mode. I tried to manually set a screenshot button in Steam’s controller configuration, (Steam > Settings > Controller > Desktop Configuration > {button} > Camera Icon), but the button seemingly did nothing.