我已经拜托了某某作为我的back up,用英语怎么说

I have already asked ( ) to back-up for me.
倘若使用于同事之间,如果感情比较好就直接放入名字,如果比较正式可以使用 Mr./Ms.
例如：I have already asked Mr.Smith to be my back-up.
I have already asked John to be my back-up.
或者礼貌一点的可以：
I have requested Mr.Smith to be my back-up
或是 I have asked John to be my back-up.
希望能够帮到你.:)