class Main
{
    void test()
    {
        TaskScheduler scheduler = TaskScheduler.FromCurrentSynchronizationContext();

        var runTask = new RunTask();
        var task = runTask.CreateTask(doSomething, scheduler)
                .ContinueWith<bool>(..., scheduler);
        task.Wait();
    }
}

public class RunTask
{
    public RunTask()
    {
    }

    public Task CreateTask(Action action, TaskScheduler scheduler)
    {
        _scheduler = scheduler;
        return Task.Factory.StartNew(() => executeAction(action), CancellationToken.None, TaskCreationOptions.LongRunning, TaskScheduler.Default);
    }

    private void executeAction(Action action)
    {
        var t1 = Task.Factory
           .StartNew(action)
           .ContinueWith<Task>(mainTaskCompleted);

        var t2 = Task.Factory
            .StartNew<bool>(waitMainTask)
            .ContinueWith(progressTaskCompleted, _scheduler);//progressTaskCompleted - show and hide progress operation in UI

        var tasks = new[] {t1, t2};
        Task.WaitAll(tasks);

        if (t1.Result.IsFaulted)
        {
            throw t1.Result.Exception;
        }
    }

    private bool waitMainTask()
    {
        var showProgress = false;
        //...calculate working time
        return showProgress;
    }
    private void progressTaskCompleted()
    {//not step here with _scheduler

        //Show and Hide IPopup window
    }
}

public Task CreateTask(Action action, TaskScheduler scheduler)
{
    // I'm assuming progressTaskStarted does UI stuff to show progress,
    // and progressTaskCompleted hides it when the work is done.

    return Task.Factory.StartNew(progressTaskStarted)
        .ContinueWith(ignoredTask => action(), TaskCreationOptions.LongRunning)
        .ContinueWith(ignoredTask => progressTaskCompleted());
}