要在ARKit中添加一个盒子到锚点，可以按照以下步骤进行：

1. 首先，创建一个ARSCNView，并设置其delegate属性为当前视图控制器。

import ARKit
import SceneKit

class ViewController: UIViewController, ARSCNViewDelegate {
    @IBOutlet var sceneView: ARSCNView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        sceneView.delegate = self
        
        let scene = SCNScene()
        sceneView.scene = scene
    }
    
    // ...
}

2. 实现ARSCNViewDelegate协议的renderer(_:didAdd:for:)方法。在此方法中，将创建一个SCNBox对象作为盒子，并将其添加到新添加的锚点上。

func renderer(_ renderer: SCNSceneRenderer, didAdd node: SCNNode, for anchor: ARAnchor) {
    guard let planeAnchor = anchor as? ARPlaneAnchor else { return }
    
    let box = SCNBox(width: 0.2, height: 0.2, length: 0.2, chamferRadius: 0)
    let boxNode = SCNNode(geometry: box)
    boxNode.position = SCNVector3(planeAnchor.center.x, planeAnchor.center.y, planeAnchor.center.z)
    
    node.addChildNode(boxNode)
}

3. 最后，在视图控制器的viewWillAppear(:), viewDidAppear(:), viewWillDisappear(:), viewDidDisappear(:)方法中，启动或停止AR会话。

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    
    let configuration = ARWorldTrackingConfiguration()
    sceneView.session.run(configuration)
}

override func viewWillDisappear(_ animated: Bool) {
    super.viewWillDisappear(animated)
    
    sceneView.session.pause()
}

完成以上步骤后，当ARKit检测到平面时，它将在相应的锚点上添加一个盒子。