SpriteKit - create sprite in Gamescene.sks and reference in another class

I know I can use the following in GameScene.swift to reference a sprite I created in the level editor:

ball = childNode(withName: "ball") as? SKSpriteNode

what if I want to reference that ball from a Ball class? Below is what I thought would work. I was wrong. In the level editor I've given the ball the custom class of Ball.

1. I've created a ball sprite in GameScene.sks and assigned it the Custom Class of Ball:

2. This is my custom Ball class. (You can ignore the contents, because it doesn't work.)

   class Ball: SKSpriteNode {
   
     init() {
       let size = CGSize(width: 35, height: 35)
       super.init(texture: nil, color: UIColor.clear, size: size)
   
     }
   
     required init?(coder aDecoder: NSCoder) {
       fatalError("init(coder:) has not been implemented")
     }
   
     func moveBall() {
       self.position = CGPoint(x: 50, y:50)
     }
   }
   

3. Then, in GameScene.swift, I'd like to use the Ball function moveBall() I defined.

   class GameScene: SKScene {
     var ball = Ball()
     override func didMove(to view: SKView) {
       ball.moveBall()
     }  
   }