CName subdomain (non CloudFlare) to business account in CloudFlare?

We have a business account configured in Cloudflare (let’s say main.com).

We have a need to “proxy” a subdomain from another organization that does not use Cloudflare (dev.com), in a way so it goes through main.com

I mean if users hit sub.dev.com → in fact it goes on Cloudflare and our server (main.com) serves responses.

We have permission to configure records for sub.dev.com in a way we need (without changing dev.com records).

I have read a lot a found this article (seems to be possible but requires 200$ plan+)

Also I found that it kind of possible to add CName record on sub.com side like this (that did not work):

CNAME shop.sub.com => main.com.cdn.cloudflare.net

Could anybody give me advise if that is possible and what would be the right way to do it?

Thanks.

If you have an existing CNAME setup on a Business Plan, you can still add SSL for SaaS (Custom Hostname) to connect an external subdomain.