How do you send a personal message to another player through ingame chat? or out of game chat?
In game you can type:
/w "[player name]" [message]or
/msg "[player name]" [message]. After they’ve messaged you once, you can just type:
/r [message] and it’ll go to whoever messaged you last.
In lobby/champion select, you can double click on their name to bring up a chat.
Note: this only works with the ones in your friends list.
Source : Link , Question Author : lostpuppydog , Answer Author : Fabián