is the only way to create a user for an azure database, to use SQL query? Azure databases are contained databases (right?) then I need to use this to create a user:
CREATE USER UserName WITH PASSWORD = 'hardPass';
ALTER ROLE [db_owner] ADD MEMBER UserName ;

is there a way to do this via portal? I found the oficial documentation saying that the only way to add a user is using a query, but maybe the doc is not updated.
Thanks.
Thats correct. Only via the Create User query. The GUI is not supported to create user and permissions for Azure sql database.
You can do that via any client (ssms) or the query editor in portal