Travel to server default map during game init (GameInstance) from c++ in unreal engine. You can get or set the server default map below "Maps & Modes" in Project Settings.

CustomGameInstance.cpp

// Example how to travel to server default map during game init.
// Use a custom game instance and override method Init from UGameInstance.
void UCustomGameInstance::Init()
{
    Super::Init();

    if (IsDedicatedServerInstance())
    {
        FString ServerDefaultMap;
        GConfig->GetString(
            TEXT("/Script/EngineSettings.GameMapsSettings"),
            TEXT("ServerDefaultMap"),
            ServerDefaultMap,
            GEngineIni);
        FString URL = ServerDefaultMap.Append("?listen");
        UE_LOG(LogTemp, Warning, TEXT("UDefaultGameInstance::Init: Travel to %s"), *URL);
        GetWorld()->ServerTravel(URL, true, true);
    }
}