[flotwig]

Looks like it's not supported on the Nano, Trinket, or Uno, which were the only devices I had laying around: https://arduino.stackexchange.com/a/39638

Otherwise, that would work fine, but not all motherboards support power-on via USB HID, most support PS/2 though.

[ThatPlayer]

You can do it on the Uno, if it's real (or close enough) Uno that uses an atmega16u2 for the USB connection. Using HoodLoader2 like that post mentions: https://github.com/NicoHood/HoodLoader2 .

Most Uno clones skip the atmega16u2 and use a cheap USB to serial adapter instead, which will not let you reprogram it as a USB HID

[rdc12]

You could also hook up a GPIO pin to the wires for the power on button