When Yeshua says that Moses wrote of Him. Is there a direct reference to a specific passage like Deuteronomy 18:15 or another passage. Or is it more of a general statement of Moses writing the Torah and Yeshua being the Torah?
Good question! In John 5:46 it is recorded that Yeshua said "For if you really believed Moshe, you would believe me; because it was about me that he wrote." Yeshua was referring to Deuteronomy 18:15. But we have to understand what Yeshua was teaching to the people who were listening, but primarily to the agitated Jews who were made aware of his words (John 5:16). It was to the Jewish agitators he spoke "You keep examining the Tanakh because you think that in it you have eternal life. Those very Scriptures bear witness to me, but you won't come to me in order to have life!” (John 5:39-40).
You see, that Yeshua was telling all that the Jews could not see what his purpose was on the earth - to provide the promised eternal life. So he charged the Jewish agitators with their error, that they also did not believe what Moshe wrote (5:47).
So, while Yeshua is never mentioned by name anywhere in the Tanakh, he is definitely alluded to in many places, including the following:
Deut 18:15 “Adonai will raise up for you a prophet like me from among yourselves, from your own kinsmen. You are to pay attention to him, 16 just as when you were assembled at Horev and requested Adonai your God, ‘Don’t let me hear the voice of Adonai my God any more, or let me see this great fire ever again; if I do, I will die!’ 17 On that occasion Adonai said to me, ‘They are right in what they are saying. 18 I will raise up for them a prophet like you from among their kinsmen. I will put my words in his mouth, and he will tell them everything I order him. 19 Whoever doesn’t listen to my words, which he will speak in my name, will have to account for himself to me. (CJB)
Yeshua Himself said He came to do HIS FATHER's will; not his own!
Luke 4: 43 "But he (Yeshua/Jesus) said, "I must preach the good news of the Kingdom of God to the other towns also, because that is why I was sent." (AENT)
Luke 8: 1. And it happened after these things that Y'shua was going around in the cities and in the villages and was preaching and declaring the Kingdom of Elohim. (AENT)
John 6: 35. Y'shua said to them, I am the Bread of Life. Whoever comes to me will not hunger. And whoever believes in me will not ever thirst. 36. But I have said to you that you have seen me, and you do not believe. 37. All who my Father has given to me will come, and whoever comes not to me I will cast outside!
38. For I came down from heaven, not to do my will, but to do the will of Him who sent me. 39. And this is His will, that of Him who sent me, that all that He has given to me I do not lose even one. 40. For this is the will of my Father, that all who see the Son and believe in him, will have life that is eternal. And I will raise him up on the last day. (AENT)
 As in the context of Miltha (the Word) or the "Manifestation" or the "Will of" YHWH coming down from heaven to do the Will of YHWH upon earth; his Spirit is of YHWH but his nephesh (soul) and body are human and temporal. This shows that Yeshua was NOT in charge; HIS FATHER WAS!
Yeshua came to proclaim the Kingdom of YHWH and to do everything his Father commanded. He tore down the fences of man's legalism. Neither he nor his Apostles ever advocated against Torah!
John 12: 44. But Y'shua cried out and said, He who believes in me, it is not in me he does believe, but in Him who sent me.  45. And he who sees me sees Him who sent me! 46. I have come as the light into this world that all who believe in me might not remain in the darkness! 47. And he who hears my words, yet does not keep them, I do not judge him, for I have not come to judge the world but to give life to the world.
48. He who rejects me and does not receive my words, there is something that judges him. The word that I have spoken, it will judge him on the last day. 49. For I have not spoken from my soul, but the Father who sent me. He gave a Commandment to me what I should say and what I should speak. 50. And I know that His commands are eternal life.  Therefore, these things that I speak just as my Father told me, thus I speak. (AENT)
 Y'shua does not bring a new Faith, but reveals the original Faith of YHWH the Father.
 Y'shua proclaims that he came to give life, then he declares that his father's Commands are eternal life! The Father's "Commands" are instruction in righteousness; this is Torah which contains Commandments, Judgments, Testimony and Witness regarding the Kingdom of Elohim. Those who enter into Covenant with Mashiyach are entering into Covenant with his Father YHWH; therefore, it is evil - an outright lie - to teach that Torah is done away with and replaced by a new theology!
 The Commands of YHWH bring eternal life. YHWH's "Commands" refers to all of His Commandments; these are one and the same as every Word that proceeds out of the Mouth of Elohim. These are not Commands only between Y'shua and his Father but they are universal for all souls who will enter into the Malchut Elohim.
 Y'shua speaks the Word of YHWH, not his own words; therefore, he conveys the revelation of YHWH according to the Ruach haKodesh without the baggage of man's religious tradition. Religion has a propensity to reinvent or interpret the Word of YHWH to make it "acceptable" to man's standards; whereas Mashiyach came not to please men, but to honor and bring Glory to his Father.
Also, take a look at 2 Samuel 7:16 which says that his kingdom is going to be forever. Whose Kingdom but YHWH Himself? (Yeshua, the Divine Messiah, the Word - John 1:1, 14 - and the "Arm" - Isaiah 53:1 along with ALL of IS 53 - is all about Yeshua!!! No one else has or ever will fill those approximately 300 Messianic prophecies!