You start dedicated server you need to start your game with -server option, here docs about it:
You can’t host dedicate server on Steam, you or user of your game will need to host it somewhere. You don’t need Steam to do that, you connect to server using IP address, but you will need service that lists IPs of servers that are online, to so called master server and Steam provides that. As i lurked in engine code OnlineSubsystem sends heartbeats (information that server is online so master server keep server entry) when you use OnlineSubsystem Session system the SteamOnlineSubsystem support that, i’m not sure if you need any code intervention to make it send hearthbeat or it does that on his own when you run a server (whatever if it’s a client host or dedicated) with just configurating SteamOnlineSubsystem.
You can test it on your single computer, start server and then client, connect to it via your IP (the private one, if you want to make it work with public IP you will need to forward the port in router configuration) or using loopback ip 127.0.0.1
As for AppID i’m not sure, my intuition tells me it should work on test one, but server list will be filled with incompatible servers of other testers ;p but again you don’t need Steam to connect, all you need is IP