How can I create my own subdomain of my github.io page?

Not sure if this belongs here, but:
I have a github pages site, norway-yv.github.io. I want to create e.g. the subdomain of that subdomain short.norway-yv.github.io for URL-shortening (like, creating short.norway-yv.github.io/trains for my train-information repository project). How can I create the short.norway-yv.github.io subdomain using github pages?

>Solution :

You can’t.

Github offer you username.github.io for your main site and username.github.io/project-name for projects.