ResolveFromCallbackProblem.cs

using Jint;
using UnityEngine;
using System;
using System.Threading.Tasks;

public class RunJs : MonoBehaviour
{
    private static Engine engine;
    
    void Start()
    {
        engine = new Engine();
    
        engine.SetValue("console",typeof(Debug));
        engine.SetValue("setTimeout", new Action<Action<int>, int>(setTimeout));


        engine.Execute(@"
            function delayResolve(delay) {
                return new Promise(function(resolve) {
                    setTimeout(function() {
                        console.log('onTimeout');
                        resolve('success');
                    }, delay);

                    // resolve('works, if not called from C# inoked function')
                });
            }

            delayResolve(1000)
                .then(res => {
                    console.log('resolved: ' + res);
                })
                .catch(res => {
                    console.log('rejected: ' + res);
                })
            ;
        ");

        void setTimeout(Action<int> callback, int delay)
        {
            Task.Delay(delay).ContinueWith(_ => callback(delay));
        }
    }
}