我解决了这个问题，只需粘贴代码就可以了functions.php 代码不会删除默认的按标题搜索。

请参见带有屏幕截图的示例

首先，添加自定义字段组Post Type is equal to Product 并添加一个具有字段标签和字段名称的新字段，如下所示

在产品编辑页面中显示的自定义字段中添加一些文本

在最后一步中，只需将代码添加到function.php 并在数组中用逗号分隔字段名称$custom_fields = array("_product_test");

function search_by_custom_field_for_admin( $search, &$wp_query ) {
    global $wpdb, $pagenow;
    $post_type = \'product\';
    $custom_fields = array(
        "_product_test",
    );

    if ( \'edit.php\' != $pagenow || !is_admin() || $wp_query->query[\'post_type\'] != $post_type ) {
        return $search;
    }

    $get_post_ids = array();
    foreach ($custom_fields as $custom_field_name) {
        $args = array(
            \'posts_per_page\'  => -1,
            \'post_type\'       => $post_type,
            \'meta_query\' => array(
                array(
                    \'key\' => $custom_field_name,
                    \'value\' => $wp_query->query[\'s\'],
                    \'compare\' => \'LIKE\'
                )
            )
        );
        $posts = get_posts( $args );
        if(!empty($posts)){
            foreach($posts as $post){
                $get_post_ids[] = $post->ID;
            }
        }
    }
    $wp_query->set(\'post__in\', $get_post_ids);
    return $search;
}
add_filter( \'posts_search\', \'search_by_custom_field_for_admin\', 999, 2 );

更改后，您可以在屏幕截图中看到搜索结果