You generally cannot turn on AFB yourself on a German ICE train
AFB is controlled by the train driver, not passengers
If you are the train driver, use the train’s driving controls and follow the ICE operating manual for that specific model
If you are a passenger, ask the train crew or driver if needed
On some ICE models, AFB is enabled through the driver’s control desk after the train is powered and ready
The exact procedure depends on the ICE series and is not publicly standardized for passengers