how to add force?

This is my code:

How can I add downward force?

 public class PlayerMovement2 : MonoBehaviour
 {
     float playerHeight = 2f;
 
     [SerializeField] Transform orientation;
 
     [Header("Movement")]
     [SerializeField] float moveSpeed = 6f;
     [SerializeField] float airMultiplier = 0.4f;
     float movementMultiplier = 10f;
 
     [Header("Sprinting")]
     [SerializeField] float walkSpeed = 4f;
     [SerializeField] float sprintSpeed = 6f;
     [SerializeField] float acceleration = 10f;
 
     [Header("Jumping")]
     public float jumpForce = 5f;
 
     [Header("Keybinds")]
     [SerializeField] KeyCode jumpKey = KeyCode.Space;
     [SerializeField] KeyCode sprintKey = KeyCode.LeftShift;
 
     [Header("Drag")]
     [SerializeField] float groundDrag = 6f;
     [SerializeField] float airDrag = 2f;
 
     float horizontalMovement;
     float verticalMovement;
 
     [Header("Ground Detection")]
     [SerializeField] Transform groundCheck;
     [SerializeField] LayerMask groundMask;
     [SerializeField] float groundDistance = 0.2f;
     public bool isGrounded { get; private set; }
 
     Vector3 moveDirection;
     Vector3 slopeMoveDirection;
 
     Rigidbody rb;
 
     RaycastHit slopeHit;
 
     private bool OnSlope()
     {
         if (Physics.Raycast(transform.position, Vector3.down, out slopeHit, playerHeight / 2 + 0.5f))
         {
             if (slopeHit.normal != Vector3.up)
             {
                 return true;
             }
             else
             {
                 return false;
             }
         }
         return false;
     }
 
     void Start()
     {
         rb = GetComponent<Rigidbody>();
         rb.freezeRotation = true;
     }
 
     void Update()
     {
         isGrounded = Physics.CheckSphere(groundCheck.position, groundDistance, groundMask);
 
         MyInput();
         ControlDrag();
         ControlSpeed();
 
         if (Input.GetKeyDown(jumpKey) && isGrounded)
         {
             Jump();
         }
 
         slopeMoveDirection = Vector3.ProjectOnPlane(moveDirection, slopeHit.normal);
     }
 
     void MyInput()
     {
         horizontalMovement = Input.GetAxisRaw("Horizontal");
         verticalMovement = Input.GetAxisRaw("Vertical");
 
         moveDirection = orientation.forward * verticalMovement + orientation.right * horizontalMovement;
     }
 
     void Jump()
     {
         if (isGrounded)
         {
             rb.velocity = new Vector3(rb.velocity.x, 0, rb.velocity.z);
             rb.AddForce(transform.up * 12 * jumpForce, ForceMode.Impulse);
         }
     }
 
     void ControlSpeed()
     {
         if (Input.GetKey(sprintKey) && isGrounded)
         {
             moveSpeed = Mathf.Lerp(moveSpeed, sprintSpeed, acceleration * Time.deltaTime);
         }
         else
         {
             moveSpeed = Mathf.Lerp(moveSpeed, walkSpeed, acceleration * Time.deltaTime);
         }
     }
 
     void ControlDrag()
     {
         if (isGrounded)
         {
             rb.drag = groundDrag;
         }
         else
         {
             rb.drag = airDrag;
         }
     }
 
     private void FixedUpdate()
     {
         MovePlayer();
     }
 
     void MovePlayer()
     {
         if (isGrounded && !OnSlope())
         {
             rb.AddForce(moveDirection.normalized * moveSpeed * movementMultiplier, ForceMode.Acceleration);
         }
         else if (isGrounded && OnSlope())
         {
             rb.AddForce(slopeMoveDirection.normalized * moveSpeed * movementMultiplier, ForceMode.Acceleration);
         }
         else if (!isGrounded)
         {
             rb.AddForce(moveDirection.normalized * moveSpeed * movementMultiplier * airMultiplier, ForceMode.Acceleration);
         }
     }
 }