Azure sql database grant select on certain tables
You need to ensure the login is added on all the levels

---- ON SERVER LEVEL
CREATE LOGIN TestUser
    WITH PASSWORD = 'ThisIsAStrongPassword!' 
GO

---- ON Master Level
CREATE USER TestUser
    FOR LOGIN TestUser
    WITH DEFAULT_SCHEMA = dbo


---- ON SQL DB LEVEL
CREATE USER TestUser
    FOR LOGIN TestUser
    WITH DEFAULT_SCHEMA = dbo
GO

GRANT SELECT ON OBJECT::dbo.Sales TO TestUser;
GO


You need to add the login on server level under the master context

then add a user to the master db under master context 

and then add the user to the sqldb and grant its rights

EDIT You cannot use the use statement, so you will have to manually switch context in SSMS