How to load a scene additive and set as ActiveScene before their Start() methode is called ?

These functions are all called before the Start() function

First Scene Load

These functions get called when a scene starts (once for each object in the scene).

  Awake: This function is always called before any Start functions and also just after a prefab is instantiated. (If a GameObject is inactive during start up Awake is not called until it is made active.)
  OnEnable: (only called if the Object is active): This function is called just after the object is enabled. This happens when a MonoBehaviour instance is created, such as when a level is loaded or a GameObject with the script component is instantiated.
  OnLevelWasLoaded: This function is executed to inform the game that a new level has been loaded.

Note that for objects added to the scene, the Awake and OnEnable functions for all scripts will be called before Start, Update, etc are called for any of them. Naturally, this cannot be enforced when an object is instantiated during gameplay.

https://docs.unity3d.com/Manual/ExecutionOrder.html