[buro9]

Try using Cloudflare and having a wildcard subdomain and then letting others CNAME to it.

You cannot do this on Cloudflare without purchasing an Enterprise offering with SaaS features.

It's a very basic thing, allow CNAME to your subdomains... but nope.

So if you want to do this, you cannot use their DNS or Registrar.

[rmkrmk]

This implies you're using their reverse proxy feature, not just plain DNS records.