add_filter(
   'mastodon_api_bookmark_reaction',
    function(
        $apply_filters,
        $ret
    ) {
        // Your code here.
        return $apply_filters;
    },
    10,
    2
);

apply_filters( 'mastodon_api_bookmark_reaction', apply_filters( 'friends_bookmarks_emoji', '2b50' ), null )