You need to look for one with a specific control knob that has positions of off or on - and usually has additional options like fan only, low heat and high heat. And then it typically has another knob for setting the thermostat temperature to achieve. It will then auto shutoff when desired temp is reached.
I’d also look for one with auto shutoff if tipped over, overheat protection, etc.
Be careful what you plug these into. These units can demand a lot of power and that might toast your smart plug. Get one rated for big demand appliances.