How can Editor Script change value in a Prefab instance ?

I have a prefab with a string call CAKE initialized to APPLE in one of its component. I copy an instance of the prefab as one GameObject in the Hierarchy window. Then I use an editor script to modify the value to BANANA before the game is started.

In the inspector window, the value is correctly updated but not in bold.

When I start the game, it returns old value Apple.

If I modify the value manually in the inspector window, and start the game, it returns good value BANANA.

How can I update correcty the value using EDITOR SCRIPT ?

 //PREFAB
 public class DecorController : MonoBehaviour
 {
     public string CAKE = "APPLE";
 }

 //EDITOR SCRIPT 
 [CustomEditor(typeof(DecorController))]
 public class DecorControllerEditor : Editor
 {
     SerializedProperty m_CAKE;
     void OnEnable()
     {
         m_CAKE = serializedObject.FindProperty("CAKE");
     }
     public override void OnInspectorGUI()
     {
         serializedObject.Update();
         EditorGUILayout.PropertyField(m_CAKE);
         serializedObject.ApplyModifiedProperties();
     }

     void OnSceneGUI()
     {
         DecorController myController = (DecorController)target;
         myController.CAKE = "BANANA";
     }
 }