The X3 panel will load data from Cloudflare based on your domain. If it doesn't work from subdomain, try to open
/panel/cloudflare.php and edit line 231:
return zones(get_domain($_SERVER['HTTP_HOST']));
Change to the below, manually entering your primary domain name.
return zones('primarydomain.com');
PS! This feature is currently slightly outdated, because Cloudflare recently introduced a maximum of 30 requests to purge from the cache from the Cloudflare API. That means X3 can only purge the CF cache for 15 pages (two requests JSON + HTML for each page). Also, this feature really does not offer much benefit any more ... X3 pages (ajax/json pages) will already be cached on Cloudflare (if your website is on Cloudflare), and because pages get cached anyway on server, they are already fast. Besides, the speed-up from Cloudflare will only apply for visitors that visit from the same geographic location.