UnityGameManagerSingleton.cs

using UnityEngine;

class GameManager : MonoBehaviour
{
   public static GameManager instance;

   [RuntimeInitializeOnLoadMethod(RuntimeInitializeLoadType.BeforeSceneLoad)]
   private static Initialize()
   {
      if (instance == null)
      {
          instance = new GameObject(nameof(GameManager), typeof(GameManager)).GetComponent<GameManager>();    
      }
   }

   private void Awake()
   {
      Debug.Log("GameManager has been created!");
      DontDestroyOnLoad(this);
   }
}