Help.. Jump by Addforce , could you help me? I want my player jump by addforce

This is my "PlayerMovement" ..

public class PlayerMovement : MonoBehaviour {

  public float _gravity = 1.5f;
  public float _yVelocity = 0.0f;
  public float _moveSpeed = 25.0f;
  public float _jumpSpeed = 40.0f;
  public CharacterController _controller;
 
  private bool _onGround;
  Animator anim;
  
  

 
  // Use this for initialization
  void Start ()
  {
     
      _controller = GetComponent<CharacterController>();
      anim = GetComponent<Animator>();
    
  }
  
  // Update is called once per frame
  void FixedUpdate ()
  {
     
        
      _onGround = _controller.isGrounded ; //Gets if the controller is grounded
      Vector3 direction = new Vector3(SimpleInput.GetAxis("Horizontal"), _yVelocity, SimpleInput.GetAxis("Vertical")); //Gets the direction
      if (direction.sqrMagnitude > 1f) direction = direction.normalized; //This prevents going faster when running diagonically
      Vector3 velocity = direction * _moveSpeed; //Multiplies the movement speed
     
     
 
      
        }
      else
     
       
 
      _controller.Move(velocity * Time.deltaTime);
      Vector3 facingrotation = Vector3.Normalize(new Vector3(SimpleInput.GetAxis("Horizontal"), 0f, SimpleInput.GetAxis("Vertical")));
      if (facingrotation != Vector3.zero)         //This condition prevents from spamming "Look rotation viewing vector is zero" when not moving.
          transform.forward = facingrotation;

if (_onGround ) { //JUMPING if (CrossPlatformInputManager.GetButtonDown("Jump")) { // Jump by AddForce script here

          }
   
   

}