User has blocked requests through HTTP

Hi,

I block external request in my wp-config.php, for security.
I know need to whitelist facebook, but not sure what url to whitelist?

I have tried the below urls, but no luck.

define('WP_ACCESSIBLE_HOSTS', 'facebook.com'); // whitelist hosts
define('WP_ACCESSIBLE_HOSTS', 'developers.facebook.com/apps'); // whitelist hosts
define('WP_ACCESSIBLE_HOSTS', 'developers.facebook.com'); // whitelist hosts

The error message I get:

Facebook returned the following error when testing your configuration.
User has blocked requests through HTTP.

https://ww.wp.xz.cn/plugins/recent-facebook-posts/