I was putting the driver in the wrong section of the config (Engine.Engine instead of Engine.GameEngine). However now I have this error on RequestReservation:
[2016.12.07-16.48.04:851][343]LogInit: SteamSockets: Socket queue 32768 / 32768
[2016.12.07-16.48.04:852][343]LogNet:Warning: Failed to init net driver ConnectURL: steam.76561198050836541:15000//Game/Maps/MainMenuLevel: SteamSockets: binding to port 0 failed (0)
[2016.12.07-16.48.04:852][343]LogNet: DestroyNamedNetDriver IpNetDriver_0 [IpNetDriver_0]
[2016.12.07-16.48.04:852][343]LogExit: IpNetDriver_0 IpNetDriver_0 shut down
[2016.12.07-16.48.04:852][343]LogBeacon:Warning: RequestReservation: Failure to init client beacon with steam.76561198050836541:15000//Game/Maps/MainMenuLevel.