add_filter( 'tiny_mce_before_init', 'fb_mce_before_init' );
function fb_mce_before_init( $settings ) {

    $style_formats = array(
        array(
            'title' => 'Download Link',
            'selector' => 'a',
            'classes' => 'download'
        ),
        array(
            'title' => 'My Test',
            'selector' => 'p',
            'classes' => 'mytest',
        ),
        array(
            'title' => 'AlertBox',
            'block' => 'div',
            'classes' => 'alert_box',
            'wrapper' => true
        ),
        array(
            'title' => 'Red Uppercase Text',
            'inline' => 'span',
            'styles' => array(
                'color'         => 'red', // or hex value #ff0000
                'fontWeight'    => 'bold',
                'textTransform' => 'uppercase'
            )
        )
    );

    $settings['style_formats'] = json_encode( $style_formats );
    return $settings;
}