AddPlayerLoopCallback.cs

using UnityEngine;
using UnityEngine.LowLevel;
using UnityEngine.PlayerLoop;

public static class AddPlayerLoopCallback
{
    // Add a callback to the PreUpdate phase
    [RuntimeInitializeOnLoadMethod(RuntimeInitializeLoadType.BeforeSceneLoad)]
    static void Setup()
    {
        var loop = PlayerLoop.GetCurrentPlayerLoop();
        for (int i = 0; i < loop.subSystemList.Length; ++i)
        {
            if ( loop.subSystemList[i].type == typeof(PreUpdate))
            {
                loop.subSystemList[i].updateDelegate += Update;
            }
        }
        PlayerLoop.SetPlayerLoop(loop);
    }

    static void Update()
    {
        Debug.Log("PreUpdate");
    }
}

See also https://gist.github.com/LotteMakesStuff/8534e01043826754344a570a4cf21002
https://www.patreon.com/posts/unity-2018-1-16336053