This is a subdomain not domain
Cloudflare doesn’t allow subdomains to be registered separately, Cloudflare will only provide their services if you can provide your own registered domains
How to get free domain ? here https://www.freenom.com/en/index.html?lang=en (don’t choose .TK)
Then add your domain here
for example as addon domain
https://infinityfree.net/support/add-your-own-domain/
Wait for DNS propagation
https://infinityfree.net/support/why-doesnt-my-domain-work/
Upload files to the corresponding htdocs folder
And when everything works then add the domain to CF
and follow there further instructions regarding NS modifications
Wait again for DNS propagation