RPC to certain player in room using their nickname/userID

This is a multiplayer game using photon.

I'm trying to implement a system where players can challenge each other while in the same room.

Ex: Player 1: presses button that sends challenge to player 2 (it could be a popup) Player 2: See's the popup and has the option to accept or decline the challenge, if declined a pop will be sent to player 1 saying his challenge has been declined. But if it is accepted the two players will be teleported to two different points in the room where they can fight.

I am trying to use RPCs and Raise Event for this system but I don't know how send the challenge to the correct player. I think I need to get the users nickname or userID and send it to them like that but I dont know how I would go about doing that.

Any advice at helps.

Thanks.