Php 如何在与wordpress的slack集成中添加新用户事件

我正在尝试为slack integration添加新事件，以便在新用户注册时获取通知。但是我的代码不起作用，请帮忙

add_filter( 'slack_get_events', function( $events ) {
$events['register_form '] = array(
    'action'      => 'register_post',
    'description' => __( 'When new user added', 'slack' ),
    'message'     => function(  ) {
        return sprintf( 'New User created successfully.' );
    }
);

return $events;
} );