Editor.cs

// MapGenerator is class on which this custom editor will be called
using UnityEditor;
[CustomEditor(typeof(MapGenerator))]
public class Edit : Editor
{
    public override void OnInspectorGUI()
    {
        base.OnInspectorGUI(); 
        // if you remove |^|(base.OnInspectorGUI(); )
        // variable in inspector may not show up properly
        
        MapGenerator mapGenerator = (MapGenerator)target;
        mapGenerator.Function();
    }
}

// another approach it is called only if there is change in the value.
// use these will be more performant
using UnityEditor;
[CustomEditor(typeof(MapGenerator))]
public class Edit : Editor
{
    public override void OnInspectorGUI()
    {
        MapGenerator mapGenerator = (MapGenerator)target;
        if(DrawDefaultInspector())
        {
           	mapGenerator.Function();
        }
    }
}