Unable to get source PO file name in Localization plugin

I’m now building a Localization plugin which is derived from ILocalizationServiceProvider and talks to our own Translation service. I copied OneSkyLocalizationService and modified it to call our service instead of OneSky. But from the plugin, there’s no way to get the source file location and file name. OneSkyLocalizationService tries to get the source file name like below:

FOneSkyLocalizationTargetSetting* Settings = FOneSkyLocalizationServiceModule::Get().AccessSettings().GetSettingsForTarget(InTargetGuid);
if (Settings != nullptr)
{
	ILocalizationServiceProvider& Provider = ILocalizationServiceModule::Get().GetProvider();
	InProjectId = Settings->OneSkyProjectId;
	InSourceFileName = FPlatformHttp::UrlEncode(Settings->OneSkyFileName);
	InExportFileName = FPlatformHttp::UrlEncode(FPaths::ConvertRelativePathToFull(InRelativeOutputFilePathAndName));
}

but unfortunately no one set Settings->OneSkyFileName so it’s always null. Is there anyone who surfaced this issue too and had a resolution?