[@robin-w Moderator 4 years ago]

the ‘,2’ might be the issue, as you only then use 1, but try this

function custom_callback($args) {
    
        $args['select_class'] = 'my-class' ;
            
       return  $args;

}
add_filter( 'bbp_get_form_topic_status_dropdown', 'custom_callback');