Unlike its modern successor, the 146 is not fitted with auto spoilers. This is indicated in the cockpit via an "INOP" label on the AUTO SPLR switch on the overhead panel. 

Throughout the approach and landing, the airbrakes and lift spoilers must be manually deployed. A normal approach should include a manual airbrake deployment when crossing the threshold, which helps to reduce the aircraft to the Vref speed, followed by a manual lift spoilers deployment as soon as possible after touchdown. 

The airbrakes and lift spoilers are both controlled via the same lever in the cockpit, and the same SPOILERS control assignments in MSFS. For optimum control, you may wish to use the TOGGLE SPOILERS or SPOILERS AXIS control assignments to control the airbrakes in flight, and then trigger the EXTEND SPOILERS button upon touchdown to deploy the lift spoilers.