If it's working it should come on when temp is below 6 °C.
If it's enabled in car config you can turn it on on the head unit. On the touch screen it's timed heating/cooling, not sure on non touch screen though.
Can also be switched on/off by remote keyfob if fitted.
Google L322 FBH (fuel burning heater) and have a read.
Or search in search bar top right.