Github Pages Subdomain

Is it possible to set up a subdomain for a github pages website if the github pages site does not have that root domain associated with it?

For example if I have website.com configured for a separate website. Is it possible to have subdomain.website.com resolve to a github pages url or does github only allow it when the apex domain is also configured with github pages?

There are so many subdomain on Netlify that builded / deployed with Jekyll.