MessageBox.Show("Как дела?", "Привет",
                    System.Windows.Forms.MessageBoxButtons.OKCancel,
                    System.Windows.Forms.MessageBoxIcon.Information
                    , System.Windows.Forms.MessageBoxDefaultButton.Button1,
                     System.Windows.Forms.MessageBoxOptions.DefaultDesktopOnly);

                
FormMsg f = new FormMsg();
f.ShowDialog();