Simple Force

public Rigidbody rigidBody;
    public KeyCode forwardKey = KeyCode.A;
    public float speed = 20;

    // Start is called before the first frame update
    void Start()
    {

    }

    // Update is called once per frame
    void Update()
    {
        if (Input.GetKeyDown(forwardKey))
        {
            rigidBody.AddForce(0, 0, speed);
        }
    }
}