when you took your stock steering wheel and boss off, you would have noticed a cable for the horn and another for the HICAS.
the hicas one is mounted to the stock boss and as said above, it judges the angle of the wheel etc.
so you either need to "backyard job" attach the hicas sensor to the new boss as it was on the stock one, or spend double what you did to get a boss kit suitable to use on HICAS car.