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

apply_filters( 'mastodon_api_default_favourite_reaction', apply_filters( 'friends_favourites_emoji', '2764' ), null )

apply_filters( 'mastodon_api_default_favourite_reaction', apply_filters( 'friends_favourites_emoji', '2764' ), $this )