Add a new sorting option to FacetWP dropdown

functions.php

<?php
function register_package_id_default_search_option( $options, $params ) { //We use this section of code (from line 2 to 12) to set up a function but not to use it
    $options['package_id'] = array( //where package_id is the sorting option Identifier
        'label' => 'Package ID', //This outputs to the user on the frontend in the FacetWP sorting options dropdown
        'query_args' => array( //This is how you build a "search" in the SQL database for a specific argument or set of rules
            'orderby' => 'meta_value_num', // this tells FacetWP that we're looking at the number value of the meta key defined on the next line
            'meta_key' => '_package_id', // this is the identifier of the custom field we're sorting by
            'order' => 'DESC', // this makes the highest number come first in results
        )
    );
    return $options;
}

add_filter( 'facetwp_sort_options', 'register_package_id_default_search_option', 10, 2 ); //This tells WordPress to use the function we created on lines 2-12