mastodon_api_account_follow

add_filter(
    'mastodon_api_account_follow',
    function (
        string $user_id,
        WP_REST_Request $request
    ) {
        // Your code here
        return $user_id;
    },
    10,
    2
);

apply_filters( 'mastodon_api_account_follow', $user_id, $request )