Cadet Electric Baseboard heater safely hard-wires for 1500-Watt at 240-Volt or 1125 Watt at 208-Volt Thermostat controls up to 5280-Watt at 240-Volt or 2640-Watt at 120-Volt Easily replaces most line voltage wall-mounted thermostats Double pole (4-wire) thermostat with positive off position 22 Amp thermostat
Absolutely not. The 18/5 thermostat wire is intended for low voltage, either millivolt or 24-volt AC thermostats intended for use with gas furnaces or heat pumps.
With line voltage heaters like the Cadet, you normally run full-power house wiring (12/2 or 10/2) from the circuit breaker to the thermostat. And then, more 12/2 or 10/2 from the thermostat to the heater(s).
There are other ways to do it, but if you want to use a 4-wire thermostat or a smart line voltage thermostat, that's how you have to do it.