Including custom fields in WordPress search

wp_cf_search.php

<?php
// Source: https://wordpress.stackexchange.com/questions/414404/how-to-include-custom-fields-in-wordpress-search/414408#414408

function include_custom_field_in_search($query) {
    if ($query->is_search) {
        $query->set('meta_query', array(
            array(
                'key' => 'release_year',
                'value' => $query->query_vars['s'],
                'compare' => 'LIKE'
            )
        ));
    }
    return $query;
}
add_action('pre_get_posts', 'include_custom_field_in_search');