Hi there! The subdomain of
github.io will always match your username. Every repository in your account is automatically hosted on
<login>.github.io by default, and you cannot change that since it could conflict with another user. So if you want the
nikhawley.github.io subdomain, you’d need an account with that username. The
<username>.github.io repository has special treatment here, since it indicates to us that’s the site you want to be served at the root of your GitHub Pages domain. Any other repository in your account then gets hosted at
<username>.github.io/<repo_name> by default, unless you add a custom domain name.
There’s more information on the types of GitHub Pages sites and now they relate to domains here: About GitHub Pages - GitHub Docs
Hope that helps!