Assuming you’re using openHAB for other things in the same environment, then I would get a plug that can be flashed with Tasmota and do the following. It’s quite involved, however!
If this is the only smart device to be used in this environment (edit: and therefore you don’t need openHAB), then follow @rpwong advice. My preference would still be a plug flashed with Tasmota, but just use the in-built Tasmota UI to do the scheduling.
And if you’ve got physical access to the device, and nothing else smart: manually programmable plugs are probably the best bet, especially if only changing the schedule rarely…