The PlayerReplicationInfo DOES have a UniqueId member variable. I am not sure it contains that specific value though. You’ll have to test that, log it directly retrieved from OnlineSubsystemSteamworks and via the PRI.
The (Pre)Login function of the gamemode has that UniqueId passed in as an argument as well and simply sets it on the PRI. Since this function is called from native code I cannot check whether this will already be the UniqueId from OnlineSubsystemSteamworks or just a “random” unique id.
If that doesn’t work you can always manually send it from client to server as well.
Could you explain why you need that info on the server?