Add user with DDL Trigger

USE [master]
GO
/****** Object:  DdlTrigger [TRG_DB_CREATE_ASSIGN_DB_OWNER]    Script 
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TRIGGER [TRG_DB_CREATE_ASSIGN_DB_OWNERROLE]
ON ALL SERVER 
FOR CREATE_DATABASE 
AS 
    DECLARE 
         @NewDBName NVARCHAR(500)
        , @SQLSTR NVARCHAR(500)
, @userName NVARCHAR(100)

set @userName = 'testuser'


SELECT @NewDBName = EVENTDATA().value('(/EVENT_INSTANCE/DatabaseName)[1]','NVARCHAR(128)')


BEGIN
SET @SQLSTR = 'USE ' + @NewDBName + ';
CREATE USER '+ @userName + ' FOR LOGIN ' + @userName +' WITH DEFAULT_SCHEMA=[dbo]'
EXEC (@SQLSTR)

SET @SQLSTR = '
USE ' + @NewDBName + ';
EXEC sp_addrolemember N''db_owner'', '''+@userName +''''
EXEC (@SQLSTR);
END


GO

SET ANSI_NULLS OFF
GO

SET QUOTED_IDENTIFIER OFF
GO

ENABLE TRIGGER [TRG_DB_CREATE_ASSIGN_DB_OWNER2] ON ALL SERVER
GO