Explanation

Step 1: On the master database, run CREATE LOGIN [App1] WITH PASSWORD = 'p@aaW0rd!' Logins are server wide login and password pairs, where the login has the same password across all databases.
Here is some sample Transact-SQL that creates a login:
CREATE LOGIN readonlylogin WITH password='1231!#ASDF!a';
You must be connected to the master database on SQL Azure with the administrative login (which you get from the SQL Azure portal) to execute the CREATE LOGIN command.
Step 2: On DatabaseA, run CREATE USER [App1] FROM LOGIN [App1]
Users are created per database and are associated with logins. You must be connected to the database in where you want to create the user. In most cases, this is not the master database. Here is some sample Transact-SQL that creates a user:
CREATE USER readonlyuser FROM LOGIN readonlylogin;
Step 3: On DatabaseA run ALTER ROLE db_datareader ADD Member [App1]
Just creating the user does not give them permissions to the database. You have to grant them access. In the Transact-SQL example below the readonlyuser is given read only permissions to the database via the db_datareader role.
EXEC sp_addrolemember 'db_datareader', 'readonlyuser';
Reference:
https://azure.microsoft.com/en-us/blog/adding-users-to-your-sql-azure-database/