Add to an built in custom inspector

Hi all,

I have a class which inherits from the VerticalLayoutGroup and adds one use variable. It, omitting the stuff not relevant to this question, looks a bit like this;

 public class AbsoluteVerticalLayoutGroup : VerticalLayoutGroup
 {
     [SerializeField]
     private bool m_useScreenSize = false;
 }
 

However the inspector being used in Unity's editor isn't picking up my new variable presumably because it's a custom editor Unity provides for the VerticalLayoutGroup. It looks like this still;

I want to be able to add my new variable to the inspector but can't see how. It feels like I should be able to just inherit from this existing inspector like this;

 [CustomEditor(typeof(AbsoluteVerticalLayoutGroup))]
 public class AbsoluteVerticalLayoutGroupEditor: VerticalLayoutGroupEditor
 {
     public override void OnInspectorGUI()
     {
         base.OnInspectorGUI();
         AbsoluteVerticalLayoutGroupEditor myScript = (AbsoluteVerticalLayoutGroupEditor)target;
         myScript.m_useScreenSize  = GUILayout.Toggle(myScript.m_useScreenSize, "Use Screen Size");
     }
 }

But I don't know what the exiting inspector is called, VerticalLayoutGroupEditor is just my guess and that doesn't exist. Any suggestions on how I might achieve this, ideally without writing the inspector from scratch?