This letter is to confirm your employment at XXX( 公司名稱). The commencement date will be on _________ (日期).
or
Welcome to join XXX (公司名稱)!!!
It is to inform you to report to work on __________ (日期).
or
It is to certify that you are selected by our company.
___________ (日期) will be your first day on board.
其實寫法有很多啦!
看你喜歡那一種~~