He who comes in the NAME of the Lord

[TonyChanYT]

Matthew 21:

9 The crowds that went before him and that followed him were shouting, “Hosanna to the Son of David! Blessed is he who comes in the name of the Lord! Hosanna in the highest!”

What was this name?

The word "name" here had multiple meanings.

First, It was often used to designate the person himself, like a pronoun. Jesus use this word to refer to himself in John 15:

21 They will treat you this way because of my name, for they do not know the one who sent me.

Matthew 10:

22 You will be hated by all for my name’s sake. But the one who endures to the end will be saved.

Second, the word "name" refers to God's attributes or character, especially his authority and power.

Exodus 20:

20 See, I am sending an angel ahead of you to guard you along the way and to bring you to the place I have prepared. 21 Pay attention to him and listen to what he says. Do not rebel against him; he will not forgive your rebellion, since my Name is in him.

Proverbs 18:

10 The name of the LORD is a fortified tower; the righteous run to it and are safe.

Third, it is the incommunicable name.

Philippians 2:

9 Therefore God exalted him to the highest place and gave him the name that is above every name,

What's the name of the father that Jesus came in?

I don't think looking for some kind of secret, properly-spelled name here is productive. The word "name" carried the above 3 meanings.

See also What does the word "name" mean in the Bible?