Add VisualElement to Default Inspector - https://forum.unity.com/threads/adding-visualelement-to-default-editor.814962/#post-5403366

TestEditor.cs

    [CustomEditor(typeof(Test))]
    public class TestEditor : Editor
    {
       public override VisualElement CreateInspectorGUI()
       {
           var container = new VisualElement();
     
           container.Add(new IMGUIContainer(OnInspectorGUI));
           container.Add(new Label("Test"));
     
           return container;
       }
     
       public override void OnInspectorGUI()
       {
           DrawDefaultInspector();
       }
     
    }