TestScript.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UIElements;

[RequireComponent(typeof(UIDocument))]
public class TestScript : MonoBehaviour
{

    private VisualElement _parent;

    private void Start()
    {
        var scroll = new ScrollView();
        scroll.style.height = 200;
        scroll.style.backgroundColor = Color.white;

        for(int i = 0; i < 100; i++)
        {
            var el = new Label("Test Label");
            scroll.Add(el);
        }

        _parent = new VisualElement();
        _parent.Add(scroll);

        GetComponent<UIDocument>().rootVisualElement.Add(_parent);
    }

    private void Update()
    {
        if (Input.GetKeyDown(KeyCode.Q))
        {
            _parent.visible = !_parent.visible;
        }
    }

}