Hello,
English is not my first language, but I hope this helps:
When you create a GitHub repository with the Makecode editor it automatically sets everything up for you to share your project as an extension to others.
To do create the GitHub repository you can click on the GitHub icon (with a cat) that is on the bottom of your Makecode project (next to the disk icon) and then click on Sign In.
It will take you to a GitHub login page where you can create an account if you don’t have one.
Good Luck
X