How to find the direction the camera is facing.

Lets say that in this picture the capsule is forward, pressing w will always move the ball forward.

moving the mouse horizontally will make the camera rotate around the ball, In the above Picture you see that the camera is looking at the ball from the left looking right, i want in this situation to make pressing w make the ball go right…
I am now using

 {
		float moveHorizontal = Input.GetAxis ("Horizontal");
		float moveVertical = Input.GetAxis ("Vertical");

		Vector3 movement = new Vector3 (moveHorizontal, 0.0f, moveVertical);

		rb.AddForce (movement * speed);
	}

to move the ball but i would like to make it so that It calculates the direction the camera is pointing and then effect the moveHorizontal and moveVertical in the code above to change the direction of the press to be orientated around the camera.
-ps C#
-PPS thank you

You have to do this (duplicate question): Move rigidbody in relation to active camera - Unity Answers