Simple Third Person Camera-Relative Movement with WASD - kind of working but need a fix

Hi, So I am trying to get my player to move by adding a force to its rigidbody. I have a third person camera script that allows me to freely move the camera around the player using the mouse. In my player movement script below, I need the movement of the player to correspond with the direction the camera is facing. So far, the player moves in the direction the camera is facing, but I cannot control it with WASD. I cannot find a way to implement GetAxis into the movement calculation. Any help is appreciated :)

using System; using System.Collections.Generic; using UnityEngine;

 public class PlayerController : MonoBehaviour {
 
     public float moveSpeed = 12;
 
     Rigidbody rigidBody;
     float moveX;
     float moveZ;
 
     bool isTouching = false;
     public float jumpPower;
 
     public Transform _camera;
 
     void Start () {
         rigidBody = this.GetComponent<Rigidbody> ();
 
         if (rigidBody == null)
             Debug.LogError ("RigidBody could not be found.");
     }
 
     void Update () {
         moveX = Input.GetAxis ("Horizontal");
         moveZ = Input.GetAxis ("Vertical");
     }
 
     void FixedUpdate () {
 
         Vector3 cameraDirection = transform.position - _camera.position; //direction to move in is equal to the Vector3 between camera and player (direction camera is facing)
         cameraDirection.y = 0.0f; //zeros the vertical aspect of movement direction to stay in the x-z plane
         if (rigidBody != null) {
             rigidBody.AddForce (cameraDirection * moveSpeed, ForceMode.Acceleration);
         }
         if (isTouching && Input.GetKeyDown("space")) { //jump
             rigidBody.AddForce(new Vector3(0, jumpPower, 0) * 50, ForceMode.Acceleration);
         }
     }
 
     private void OnCollisionEnter(Collision collision) {
         isTouching = true;
     }
     private void OnCollisionExit(Collision collision) {
         isTouching = false;
     }
 }