The fifth parameter to MessageBox.Show indicates the default button. The following code shows how the program displays a MessageBox with the second button as the default so it is initially selected when the message box appears.

// Display a message box with the second button the default.
private void btnButton2_Click(object sender, EventArgs e)
{
    DialogResult result = MessageBox.Show(
        "The second button is the default.",
        "Caption",
        MessageBoxButtons.YesNoCancel,
        MessageBoxIcon.Information,
        MessageBoxDefaultButton.Button2);
    MessageBox.Show("You selected " + result.ToString());
}