First log into sql server using sql management studio or other relevant tool as an administrator.
Then use following command to create testuser in testdb database. You can ignore DEFAULT_DATABASE part as well if it is not working.
-- Create user for SQL Authentication
USE master
CREATE LOGIN testuser
WITH PASSWORD = 'test#123',DEFAULT_DATABASE = testdb
GO
-- Creates a database user for the login created above.
USE testdb
CREATE USER testuser FOR LOGIN testuser
GO
