TStringBuilder<512> Message; // Reserve memory on the stack
Message << TEXT("This message is exactly ");
Message.Appendf(TEXT("%d"), 512);
Message << TEXT(" characters long.");
Message.Appendf(TEXT("\n"));
UE_LOGFMT(LogTemp, Warning, "{Message}", Message);
GEngine->AddOnScreenDebugMessage(-1, 25.f, FColor::Yellow, Message.ToString());
Use << for simple, non formatted texts (but always make sure to use TEXT()).
Use .Appendf(TEXT()) for formatted texts.
To convert to an FString, simply use .ToString().