Slow Character Movement

Hey everyone,

I have been trying to make my character slow down when the right mouse button ("Fire2") is pressed, as this button allows the player to zoom and aim at an object. I have tried to add an IF Statement but still would not work, can anyone help me with this issue?

This is the player movement script I have wrote by following ETeeskiTutorials on YouTube.

Thanks a lot for the help guys, really appreciate it.

 #pragma strict
 
 //Movement Variables
 var WalkAcceleration : float = 5;
 var WalkDeacceleration : float = 5;
 var MaxWalkingAcceleration : float = 20;
 
 @HideInInspector //Hide next line from inspector
 var WalkDeaccelerationVelocityX : float;
 @HideInInspector //Hide next line from inspector
 var WalkDeaccelerationVelocityZ : float;
 @HideInInspector //Hide next line from inspector
 var HorizontalMovement : Vector2;
 var MainCamera : GameObject;
 
 function Update () 
 {
     //Get and save the velocity of X and Z axis as rigidbody - MaxWalkingAcceleration
     HorizontalMovement = Vector2(rigidbody.velocity.x, rigidbody.velocity.z);
     
     //Magniture function returns the length of this vector - MaxWalkingAcceleration
     if (HorizontalMovement.magnitude > MaxWalkingAcceleration)
     {
         HorizontalMovement = HorizontalMovement.normalized; 
         HorizontalMovement *= MaxWalkingAcceleration; //Set speed to MaxWalkingAcceleration
     }
     
     //Apple Horizontal moveemnt to rigidbody - MaxWalkingAcceleration
     rigidbody.velocity.x = HorizontalMovement.x;
     rigidbody.velocity.z = HorizontalMovement.y;
     
     //De-accleration when Character is not moving - WalkDeacceleration
     if (Input.GetAxis("Horizontal") == 0 && Input.GetAxis("Vertical") == 0)
     {
         //Decrease speed to WalkDeacceleration smoothly
         rigidbody.velocity.x = Mathf.SmoothDamp(rigidbody.velocity.x, 0 , WalkDeaccelerationVelocityX, WalkDeacceleration);  
         rigidbody.velocity.z = Mathf.SmoothDamp(rigidbody.velocity.z, 0 , WalkDeaccelerationVelocityZ, WalkDeacceleration);
     }
         
     //Rotation Y was inherited from MouseLook script - WalkAcceleration
     transform.rotation = Quaternion.Euler(0, MainCamera.GetComponent(MouseLookScript).CurrentYRotation, 0);
     
     //Horizontal Value was changed from "Input.GetAxis("Horizontal") * WalkAcceleration" to 0 - WalkAcceleration
     rigidbody.AddRelativeForce(0, 0, Input.GetAxis("Vertical") * WalkAcceleration);
     
 }