I’ve tried the code below. But this just returns the Full toolbar without the desired extra dropdown menu.
add_filter( 'acf/fields/wysiwyg/toolbars' , 'my_toolbars' );
function my_toolbars( $toolbars ) {
$style_formats = array(
array(
'title' => 'My style',
'block' => 'span',
'classes' => 'mystyle',
'wrapper' => true,
),
);
$toolbars['My toolbar'] = $toolbars['Full'];
$toolbars['My toolbar'][1]['styleselect'] = json_encode( $style_formats );
return $toolbars;
}