OK, the idea is to get the PlayerName from the OnlineSubsytem (for example steam), it has method to request that on the client machine, or just simply store it somewhere for the client (a savegame, a config, whatever). So in the PostLogin call you just call a client RPC on the PlayerControler, once on the client you just ping back the desired PlayerName with a Server RPC back to the host. The host will then ultimatly just set the PlayerName of the Players PlayerState so it will get replicated to the full host of connected players (you could skip the owner for that var and set it on the client to save it to replicate).