From my experience of my D3, the FBH can only be triggered by a timer module from Webasto, called the oval timer. Or it can be controlled by a remote module available as a kit. There are no factory fitted remotes or timers, but this is obviously for the D3. The D4 will probably be similar though, as the FBH is controlled by a W bus. There is no simple 12v switch line like the thermotop C FBH.
I have to ask what you want the FBH to do? In my experience all it does is warm part of the heater circuit. The heater doesn't function, so the cabin stays cold until you start the engine and give it time to warm up. I also found that the FBH warmed the heater circuit part of the engine nicely, but the moment the engine started, that heat was lost to cold parts of the engine, so kinda making preheating the engine a pointless exercise, especially as the cabin wasn't heated.