I can't get my Unity player to move in the direction it is facing using Rigidbody

Question

    public Rigidbody playerBody;
    public float speed = 5f;

    void Update()
    {

        float horizontal = Input.GetAxis("Horizontal");
        float vertical = Input.GetAxis("Vertical");
        playerBody.velocity = new Vector3( horizontal * speed, playerBody.velocity.y, vertical * speed);

    }

I cant get the character to move in the direction the camera is facing.

Answer 1

Thanks for your help, but I tinkered with it and got it done. Here is the code.

playerBody.velocity = transform.forward * speed * vertical + transform.up 
                      * playerBody.velocity.y + transform.right * speed * horizontal;

Answer 2

Try using MovePosition instead of velocity

playerBody.MovePosition(new Vector3( horizontal * speed, playerBody.velocity.y, vertical * speed));