This HomeKit-compatible smart outlet is well-priced and works great. But if you have to reset to factory defaults (in order to move it to a different home, for example) it can be challenging to figure out how to reset it.
The Meross website only has generic resetting instructions, and the way they are written it seems like they don’t apply to this device. Here, they say to hold down the power button. (The device has two power buttons.) Elsewhere on their site, the answer is illustrated by showing a non-existent button on the back of the device.
The correct answer, specifically for the Meross MSS120 Outlet, is to hold down the top power button for five seconds (while the outlet is plugged in). You’ll hear a click, and then the two power buttons will being to flash (one green, one yellow). You can then add the device to HomeKit.