How do you manually turn on (or off) a Mill heater in Home Assistant?

How do you manually turn on (or off) a Mill heater in Home Assistant?

The obvious thing to do (for me at least) is to press the “Heat” button on the Thermostat card for the Mill heater, but that just gives a popup message “failed to call service climate/set_hvac_mode”. The heater in question is Mill gen 3, local access. I’m running Home Assistant 2022.11.5 in a venv.