ui-toolkit-manual-code-examples/create-a-tabbed-menu-for-runtime/TabbedMenu.cs

using UnityEngine;
using UnityEngine.UIElements;

//Inherits from class `MonoBehaviour`. This makes it attachable to a game object as a component.
public class TabbedMenu : MonoBehaviour
{
    private TabbedMenuController controller;

    private void OnEnable()
    {
        UIDocument menu = GetComponent<UIDocument>();
        VisualElement root = menu.rootVisualElement;

        controller = new(root);

        controller.RegisterTabCallbacks();
    }
}