How to Make First Person Character Sprint?

I’ve been trying to make my character sprint when i hold down left shit. i’ve just been adding onto unity’s default first person character but when i hold down shift nothing happens (btw i’ve added i custom input) This is what i’ve added on:

if (Input.GetButton("Shift")) {
    maxForwardSpeed = 18  ;
    maxSidewaysSpeed = 14 ;
}

else  {   
    maxForwardSpeed = 9  ;
    maxSidewaysSpeed = 7 ;                      
}    

also everyone probably thinks I’m stupid for not knowing this but im new to unity and Javascript

How to Make the FPS Character Controller RUN and CROUCH