Https://github_account.github.io/desiredname.github.io

Hi!

First I have followed the exact instructions, https://pages.github.com/
to create a github page. I named my repository, e.g., desiredname.github.io. However, after pushing the index.html file to the repo, and setting >> GitHub Pages, the published url is showed as: https://mygithubaccount.github.io/desiredname.github.io. I can rename it from the setting, but still the url is https://githubaccount.github.io/afterrename/

I actually want to have the most convenient url, something like https://mydesiredname.github.io. If someone could suggest any solution, I would really appreciate.

p.s., in another repository, I tried to import the Jekyll Academic template. All is working fine, except again the url conflict, I can so far use the url as https://mygithubaccount.github.io/desiredname. I tried to update the _config.yml file to modify the url, but it did not work, or may be I do not know how to do it.

How can I have an url https://desiredname.github.io?