Including custom fields in string search

I would like the string search to also search some custom field contents. Apparently, only title and content of the posts are searched.

I have some filters in my functions.php that does this for default wp searches, but it isn’t working with WP query search filter (the filters use these hooks: posts_join, posts_where, posts_distinct).

By the way, this plugin is really good, much better than other filter plugins. Thank you.

https://ww.wp.xz.cn/plugins/ultimate-wp-query-search-filter/