sorry, “base_game_app_id” is the actual id, “SteamAppId” is the key to the env var, so it would look like:
FPlatformMisc::SetEnvironmentVar(TEXT("SteamAppId"), TEXT(##your_game's_actual_base_game_app_id##));
FPlatformMisc::SetEnvironmentVar(TEXT("SteamGameId"), TEXT(##your_game's_actual_base_game_app_id##));