If you own the base domain, you can add it to your InfinityFree account using the CNAME method and then host just your chosen subdomain(s) of it on InfinityFree using “A” records.
If you don’t own the base domain, you can only do so if it’s in the public suffix list, such as pp.ua. If the answer to both of those questions is “no”, then I’m afraid you can’t host the subdomain here. InfinityFree offers free subdomains of its own that you can use, though.