Oh, server can use client API

thx for your help! I am actually using unity's matchmaking and multiplay, can I know which S2S api to use to query the player details?

Hi, if I have a dedicated server that the (client)players joins for a match, if the client logs in with Client API Authenticate on their machine and I assume the client will send their profile id to the server.
Then, how can the server get the user name of all the client that are connected with their profile ids?