How to use the babylonjs-gui.StackPanel3D function in babylonjs-gui

var grassMaterial = new BABYLON.StandardMaterial("grass", scene);
    var grassTexture = new GrassProceduralTexture("textgrass", 256, scene);
    grassMaterial.ambientTexture = grassTexture;
    grassMaterial.diffuseTexture= grassTexture;

    // Create Ground
    var ground = BABYLON.Mesh.CreatePlane("ground", 25.0, scene);
    ground.position = new BABYLON.Vector3(0, -5, 0);
    ground.rotation = new BABYLON.Vector3(Math.PI / 2, 0, 0);

    ground.material = grassMaterial;
 
    // Create the 3D UI manager
    var manager = new GUI.GUI3DManager(scene);
    // Create a horizontal stack panel
    var panel = new GUI.StackPanel3D();
    panel.margin = 0.02;
    manager.addControl(panel);
    startGameButton(panel);
    addLabelToScene();
    // Enable VR
    var helper = scene.createDefaultVRExperience({createDeviceOrientationCamera: false});
	helper.enableInteractions();

    return scene;
}