Subdomain on Cloudflare?

[JxstErg1]

You can’t add our free subdomains to Cloudflare in any way. You’ll be better off getting a custom domain, and here are some sites to get one for free:

Where do I get free domains?

You can try using EU.org or PP.UA (the latter one if you have a credit card even with a few cents on it and a phone number to use for verification, though the credit card is used only once for verification and then what’s free will remain free) as a valid alternative to Freenom (and Dot TK because Dot TK and Freenom are the same) that also supports Cloudflare and other things that Freenom with its domains supported.