i’m trying assign custom claims using an onCall function but i get a CORS error.

I am working on a project, where I want the users to able to view each others profiles.
I want the the url to be like github, so my-project.com/username . To achieve that I want to set a custom uid for everyuser. I don’t want the uid to random characters like it is by default in firebase authentication. I want it to be a unique and readable username .