how to display Remaining Minutes and seconds using this Timer

private void CountdownTimer_Load(object sender, EventArgs e)
        {
            Timer MyTimer = new Timer();
            MyTimer.Interval = (20 * 60 * 1000); // 20 mins
            MyTimer.Tick += new EventHandler(MyTimer_Tick);
            MyTimer.Start();
        }
 
        private void MyTimer_Tick(object sender, EventArgs e)
        {
            MessageBox.Show("time is up");
            this.Close();
        }
 
Timer MyTimer = new Timer();