Dr. Martin Luther continues his commentary on Deuteronomy 18 verse 16: “Just as you desired of the Lord your God at Horeb” – translated by Richard R. Caemmerer in Luther´s Works Volume 9, pages 180-182:

We read nothing about any such prayer in Ex. 20. Why, then, does Moses say this? Answer: Moses infers from the very words of the people that this request was in them when they said: “I will not further hear the voice of the Lord my God, and I shall not see this mighty fire any more, lest I die.” From these words, I say, he takes, not what they say but what they desire; for he looks at their inmost feeling and at the core of their desire. They had been so moved by terror of the voice that with the greatest ardor of the heart they longed for a gentler and more pleasant word. Death itself, being held before them through the voice of the Law, forced them to sigh for life and for the Word of salvation; and in such anxiety they rather wish than dare to pray for another word. But God, who searches the depths of the heart (Acts 15:8), hears and answers, not according to the outward sound of the words but according to the feeling burning inside, just as in the Gospel Christ looks upon Zacchaeus, not according to what he was saying in public but according to the feeling in his heart, Luke 19:2 ff. Zacchaeus would not have dared beg publicly that Christ enter his home, yet inwardly he wanted nothing more than he wanted this. Therefore he also received Christ with joy, since his desire, which he himself barely sensed before was fulfilled.
In this passage Moses shows with what fervor and longing of the heart the Word of grace should be desired. It is truly that gift of God about which Paul seems especially to be speaking when he says to the Ephesians (3:20): “God is able to do far more abundantly than all that we ask or think.” He hates lazy and cold petitioners, who hope to gain their end through much speaking (Matt. 5:7); he wants sighings that cannot be uttered (Rom. 8:26). Certainly no one can have all this who spends his days in a good life and dwells in the land of luxury. This is only for those who are moved to despair by the feeling of death and the bite of sin, so that, like the people of Israel, they refuse to go on hearing the voice of God or seeing the mighty fire, that is, the power of the Law and the prick of sin, namely, death and the wrath of God, which already consumes them like a fire, so that they long to hear the Word of life.
Note how beautifully Moses yields his ministry to the future Prophet. He says that the people asked for another prophet to be raised up, although in Ex. 20:19 they asked that Moses himself speak, but said nothing of another one to be raised up besides Moses. With these words Moses indicates that because of anxiety the people had not known what they were asking for or how they were praying. It was stupid to ask to hear Moses, when Moses had to teach or proclaim nothing else than what they themselves heard at the mountain, that is, the Law. Thus frightened and anxious people tend not to know what they are praying for, as Paul says Rom. 8:27: “We do not know what we are to pray or how we ought to pray, but the Spirit intercedes for the saints, etc.” Therefore here, too, Moses interprets their request according to their inner feeling, as though he were to say: “It was stupid of you to ask that I speak to you, since you were afraid to hear that voice on the mountain and this is the voice of my ministry forever. But God, who looks at your heart, sees that you desire not me but another, who would speak more gently than I. And in answer to this request He will give you the kind of Prophet you want.
“But what kind do you want? Certainly one who heals those who are smitten through my voice, that is, through the Law of sin and wrath; who makes sound those who are contrite of heart; who frees and consoles those who are frightened and broken. With what medicine? Certainly with the Word of life and salvation, that you might hear the voice of those who proclaim peace and good things on the mountains (Is. 52:7). You want the One of whom Malachi later foretold (3:1): ‘And suddenly will come to His temple the Messenger of the covenant, whom you desire’; and after him Isaiah (61:1–2): ‘The Spirit of the Lord is upon Me, because He has anointed Me and sent Me to proclaim the Gospel to the poor, to heal the contrite of heart, to preach the year accepted by the Lord.’ Such a One you asked for in Horeb when you stupidly asked me; and such a One the Lord will therefore give you in the place of me. See that you hear Him instead of me. To Him I shall yield, because you cannot bear me, and you have asked that I be silenced and put aside.”