try this in your cursor code

DECLARE @SQL NVARCHAR(4000);

While

begin

SET NOCOUNT ON

SET @SQL = 'CREATE LOGIN ' + @username + ' WITH PASSWORD = ''12345'', DEFAULT_DATABASE=[dbname], DEFAULT_LANGUAGE=[British], CHECK_EXPIRATION=OFF, CHECK_POLICY=ON';

EXECUTE(@SQL);

exec sp_addrolemember db_datareader, @username ;

exec sp_addrolemember db_datawriter, @username ;

end