Dude, you need their number id, not their username...

oh....thanks,but how do i get his number ID

Go to your profile, then match history, click on their name and profile, next to their name will be a copy of their ID (ie. tap on the player's profile picture).

I tried it but nothing happens?

i

If not you can also go into Friends List, go to Recommended which shows people from previous battles and click on their name as well.  It will bring up their profile and show ID.