Thats the thermo switch for the heatable spraying nozzle, it turns on the heated spray nozzles if your car has them (if you get ice on your windshield you can hit that stuff and it will de-ice it quicker than chipping it off). There should be a bracket to hold it on near the tank, if not then you can get a bracket from BMW.
Bracket part number is 61318359077
Number 24 is the heat switch
You want #25 the bracket for it