The claim is that in order to identify what is good and what is bad, one must look at the idea of morality, no?
What does that have to do with validity??

In order to identify what is good and what is bad, one observes, and good and bad receive their validity by virtue of whoever legislated them. See the fourth booklet.

I didn't find an explanation there.
Indeed, the Rabbi goes on at length that one cannot know what is good and what is bad without an external source (with which I agree),
and from there jumps to the conclusion that there has to be someone who gives them their validity.

But my question is why can't one say that morality is like an ancient law book? (Instead of claiming that it requires legislation.) I didn't see any discussion of that there.

I tried to decipher the message hidden in this riddle-like wording. How does an ancient law book help you? What validity does it have? Because it is ancient, it is binding? What is the meaning of this pilpul?

What he wants to say is that there is only evidence that morality exists. Its validity is part of what one sees (otherwise it wouldn't be morality but something else, aesthetics for example). The inference from here to the existence of an entity is not necessary.

If you see something, that is a sign that there is something to see, meaning that at the basis of morality there exists an entity. And its primordial nature is of course not relevant in any way to the discussion.

In the end, whether behind the law there stands a legislator or an ancient law, the one who accepts the yoke of the law is the human being. So why is it preferable to assume that behind the law there is a legislator?

Obviously, if you accept it, then you accepted it. The question is why accept it, and whether it is reasonable to judge someone who does not accept it. You can take upon yourself the rules of the Indochina Shoe-Shiners' Association if you want. But morality is supposed to have a justification or validity. This is not an arbitrary acceptance. All this was explained in the fourth booklet.

And how does that mysterious figure give more validity than the lawmakers who enacted the regulations of the Indochina shoe association?

Granted, if that same figure who changed morality had also created the world, then fine…
But just some mysterious figure that popped up out of nowhere and started writing what is good and what is bad? That's who we have to listen to?!
But by contrast, in the shoe association, myriads upon myriads of shoe-shiners accepted the exclusive authority of the lawmakers of New Delhi-Beijing who labored over writing the laws.
So true, one day the community of shoe-shiners can rise up and turn their backs on the lawmakers and the laws, but as long as the masses have not done so, the individual is subject to it because of the masses' acceptance.

But morality? One ought to know who that anonymous one is who wrote it. By default, as long as he is not the one who created the world, who would regard him as possessing all that much authority to validate anything?! Therefore the obligation of morality also comes from the acceptance of the masses! Of the laws of that anonymous so-and-so moral legislator. (So indeed this lies on the seam between subjective-national morality and objective morality: the moral book really exists in reality, but my obligation toward it comes from the fact that the masses accepted it upon themselves.)

But none of the above follows at all from the Rabbi's words. I would be glad if the Rabbi would shed light on what he thinks about this. Who is that anonymous legislator whose voice we obey in order to fulfill the words of morality? Just some random fellow up above? Or perhaps indeed one of the honored ones of heaven…
And Pharaoh already asked a similar question — as long as it is not known to us who that legislator is, even if he indeed exists there is no reason to listen to him by default:
"And Pharaoh said, Who is the Lord that I should obey His voice to let Israel go? I do not know the Lord, and moreover I will not let Israel go…" (Only after the signs and wonders, when one sees that the Lord is the creator of the world, is there already room to listen.)

You tell me what gives it validity. As I explained in the fourth booklet, the argument is addressed to someone who thinks that the laws of morality have validity. If you think they do not, then the argument is not relevant for you. But if there is validity, then necessarily there is something that gives that validity. Everything is explained in the booklet, and I see no point in continuing here what I already detailed there.

What I'm arguing here is very clear, even if we assume that I do see validity in morality.
Is it enough for me to assume that there exists some anonymous entity that legislated it? Or must I assume that he also created the world? That is my question.
Because if I say it's just some somebody, what validity does that give me? Can the governor of the Sinai district tell me what to do?! Clearly it must be a more significant figure. Like someone who creates the world.

Does the Rabbi agree with that or not necessarily? (And can the moral legislator be just anyone, even some random fellow up above…)

You declare that you have a clear argument and then present a question.
As I wrote, the discussion is empty of content. Please, ask yourself: if you see validity in morality, what do you think gives it that validity? That's all. If you want him to have created the world — health to you. And if not — that's fine too. What does that have to do with the discussion and the argument? If there is validity, there must be something or someone that gives it that validity. That's all.