PreUpdateSystem.cs

using UnityEngine;
using UnityEngine.Experimental.LowLevel;
using UnityEngine.Experimental.PlayerLoop;
using System.Collections.Generic;

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

        var playerLoop = PlayerLoop.GetDefaultPlayerLoop();

        var updateSystem = playerLoop.subSystemList[4]; // Update
        var subSystem = new List<PlayerLoopSystem>(updateSystem.subSystemList);
        subSystem.Insert(0, mySystem);
        updateSystem.subSystemList = subSystem.ToArray();
        playerLoop.subSystemList[4] = updateSystem;

        PlayerLoop.SetPlayerLoop(playerLoop);
    }

    public struct MyLoopSystemUpdate{ }
}