This should be at the bottom of your ini file:
[/Script/Engine.GameEngine]
!NetDriverDefinitions=ClearArray
+NetDriverDefinitions=(DefName="GameNetDriver",DriverClassName="/Script/OnlineSubsystemSteam.SteamNetDriver",DriverClassNameFallback="/Script/OnlineSubsystemUtils.IpNetDriver")
[OnlineSubsystem]
DefaultPlatformService=Steam
PollingIntervalInMs=20
bHasVoiceEnabled=false
[OnlineSubsystemSteam]
bEnableSteam=true
bEnabled=true
SteamAppId=480
SteamDevAppId=480
GameServerQueryPort=27015
bRelaunchInSteam=false
P2PConnectionTimeout=90
bAllowP2PPacketRelay=true
bVACEnabled=1
GameVersion=1.0.0.0
[/Script/OnlineSubsystemSteam.SteamNetDriver]
NetConnectionClassName="/Script/OnlineSubsystemSteam.SteamNetConnection"
[Voice]
bEnabled=false
I believe the app ID must be changed later when you upload to steamworks. Playing straight from a packaged game shows up on the steam overlay as ‘spacewar’