Parallel forEach search file

I have a program that contains 2 listbox, this program is bassed to search file, and then compare with a stopWatch the difference to use AsyncAwait and TPL... The first listbox does the function using AsyncAwait (I don't know if it's the better way to do but it works, see my code below)

 private async void button1_Click(object sender, EventArgs e)
        {
            Stopwatch stopWatch = new Stopwatch();
            foreach (string d in Directory.GetDirectories(@"C:\Visual Studio Projectes\Hash\AsyncAwait\Carpetes"))
            {

                foreach (string s in Directory.GetFiles(d))
                {

                    stopWatch.Start();
                    listBox1.Items.Add(s);
                    await Task.Delay(1);
                    btIniciar1.Enabled = false;
                }

            }
            btIniciar1.Enabled = true;
            stopWatch.Stop();
            TimeSpan ts = stopWatch.Elapsed;
            textBox1.Text = ts.ToString("mm\\:ss\\.ff") + (" minuts");



        }

And then in my second listbox is where I'm stuck, I don't know how to implement the Parallelism.ForEach to act like async, what's the better way to do this? I can't find the way to use TPL in this case to do the same as my first listbox, could you help me please?

Thanks.