It is unclear if user sign up is only available via only interactive client (when user can either login or sign up their own account).
But the management API “create user” looks like it is for “Admin” to create new user but not via CLI. Can user sign up be done via CLI?
Another alternative is to do user signup using the /dbconnections/signup endpoint of the Authentication API. See Authentication API Explorer
That can easily be called using CURL or anything that can make HTTP requests.