On my hosting, 4424club.xyz is an addon domain from the subdomain 4424club.xyz.412quack.com. Am I able to also cover the subdomain, as I can only use the addon domain. Do I have to add 412quack.com to Cloudflare as well?
If your hosting is configured to serve that xyz domain with an assigned IP address, Cloudflare can proxy that xyz domain. Addon domains should behave like any other domain with regular DNS.
No need to add the quack domain, as it’s (probably) not on Cloudflare.