If your DeLonghi heater has an electronic digital display,  NO, you cannot use a Smart Plug as it will not be able to turn on the unit.
If your unit is "Non Digital" meaning it has a dial or knob you can set to turn it on, then YES you can set it to the ON position and control it using a Smart Plug schedule.    Please note: the wattage of the Smart Plug MUST be suitable to accommodate the wattage  of the unit you are using it on.