SpawnSystem.cs

using UnityEngine;
using UnityEngine.Experimental.LowLevel;

public class SpawnSystem
{
    [RuntimeInitializeOnLoadMethod()]
    static void Init()
    {
        var mySystem = new PlayerLoopSystem()
        {
            type = typeof(MyLoopSystemUpdate),
            updateDelegate = () =>
            {
                Debug.Log("DO IT!!!");
            }
        };

        var playerLoop = new PlayerLoopSystem();
        playerLoop.subSystemList = new PlayerLoopSystem[] { mySystem };
        PlayerLoop.SetPlayerLoop(playerLoop);
    }

    public struct MyLoopSystemUpdate{ }
}