protected bool VerifyPIN(string username, string PIN)
{
    return PIN.Equals("12345")
}

protected void SubmitButton_Click( object sender, EventArgs e )
{
    try
    {
        if(!VerifyPIN(UsernameTextBox.Text, PINTextBox) {
            ErrorTextLabel.Visible = true;
            ErrorTextLabel.Text = "Wrong PIN"
            return;
        }

        // PIN was ok, let's continue with SignIn
        SignIn( UsernameTextBox.Text, PasswordTextBox.Text );
    }
    catch ( AuthenticationFailedException ex )
    {
        HandleError( ex.Message );
    }
}