You can’t add subdomains on Google AdSense. You must have your own Top-Level Domain (TLD) name. If you don’t know how to get one, get one free from Freenom (but note that .tk domains aren’t supported on free hosting, the other types are), point it to our nameservers, add it on the “Parked Domains” section of the Control Panel and change your WordPress site and home URLs to contain your own domain name in place of the old one.