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