Form2.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace NonBlockingMessageBox
{
    public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            MessageBoxWrapper.Show(this.Handle, "I will block you!", "Blocking");
        }

        private void button2_Click(object sender, EventArgs e)
        {
            MessageBoxWrapper.Show(IntPtr.Zero, "I will not block you!", "Non-Blocking");
        }
    }
}