Add Custom Classes to the styleselect Dropdown from the WYSIWYG Editor

Wordpress - WYSIWYG custom classes

/**
 * Custom Classes for the wordpress WYSIWYG editor
 */

add_filter( 'tiny_mce_before_init', 'custom_mce_before_init' );
function custom_mce_before_init( $settings ) {
  $style_formats = array(
    array(
      'title' => 'Title for style',
      'selector' => 'li',
      'classes' => 'special',
      'wrapper' => false,
    )
  );
  $settings['style_formats'] = json_encode( $style_formats );
  return $settings;
}