The first thing to confirm is whether you want a page or a subdomain.
The name of a page shows up at the end of a domain name, after the TLD or extension, e.g. mydomain.com/blog -- "blog" is the name of the page.
A subdomain shows up at the beginning of a root/naked domain, e.g. blog.mydomain.com – "blog" is he subdomain.
You can create subdomains in your iwantmyname dashboard. The subdomain, e.g. "blog" does in the Hostname field.)
You cannot create pages in the dashboard. Pages are created in your hosting account, whether that's in WordPress or Squarespace or another service/platform.
You can add subdomains, e.g. blog.mydomain.com in two ways in your iwantmyname account.