You need to have a domain that you manage DNS for in Cloudflare. Look up what a "registrar" is, a common one people go through would be Namecheap. Get a domain, and then look up how to set up a DNS zone in Cloudflare from an external registrar. If you plan on working in tech, this is one of those things you'll absolutely need experience with doing. Good luck!

Though it occurs to me their may just be a language barrier and you may have a domain that you manage your DNS in Cloudflare already. If that's the case, a subdomain is just an A record under your domain's DNS settings for anything other than the root domain. So, if your domain is "example.com", the A record could be like "service" with an IP of "192.168.1.10", and your subdomain would then be served on "service.example.com" for example. Subdomains are free, if you have a domain in the first place.

If you're asking if you would already need the subdomain configured in your DNS settings in Cloudflare, then yes, most likely. Though there are tools that create those for you, like external-dns in kubernetes.