Add style formats to WordPress editor dropdown

functions.php

function my_mce_before_init_insert_formats( $init_array ) {
	$style_formats = array(
		array(
			'title'   => 'Headline',
			'block'   => 'h2',
			'classes' => 'headline',
			'wrapper' => false,
		),
		array(
			'title'   => 'Blockquote',
			'block'   => 'blockquote',
			'classes' => '',
			'wrapper' => false
		),
		array(
			'title'    => 'Button link',
			'selector' => 'a',
			'classes'  => 'button',
			'wrapper'  => false
		),
		array(
			'title'    => 'CTA Button link',
			'selector' => 'a',
			'classes'  => 'cta',
			'wrapper'  => false
		)
	);
	// Insert the array, JSON ENCODED, into 'style_formats'
	$init_array['style_formats'] = json_encode( $style_formats );

	return $init_array;
}
add_filter( 'tiny_mce_before_init', 'my_mce_before_init_insert_formats' );