Every single one of God's prophecies recorded in the Bible has been fulfilled, and Joshua must fulfill every prophecy that applies to you. In conclusion, when Joshua says, "It is finished," he is referring to the fulfillment of a prophecy that he will fulfill while in the flesh-the fulfillment of the prophecy about the Passover lamb.

[John 19:28~30] Later, knowing that all was now completed, and so that the Scripture would be fulfilled, Jesus said, "I am thirsty." A jar of wine vinegar was there, so they soaked a sponge in it, put the sponge on a stalk of the hyssop plant, and lifted it to Jesus' lips. When he had received the drink, Jesus said, "It is finished." With that, he bowed his head and gave up his spirit.

God sent a prophet named Moses to free Israel from slavery in Egypt, but this was not just a history of that time, but a prophecy that would be fulfilled beginning with Jesus.Joshua spoke words of truth and power to Judaism, which was represented by Egypt, and brought spiritual plagues upon them. This caused much uproar in Judaism, and the leaders of Judaism were afraid that the Jews would abandon Judaism after seeing the words and power of Joshua. So the words and power of Joshua were a disaster for Judaism.

However, Joshua's words and power alone were not enough to free spiritual Israel from Judaism, so his sacrifice on the cross was a fulfillment of the prophecy of the Passover observed in Egypt. To free Israel from Egypt, God brought the plague of killing the firstborn in Egypt, but Israel was able to avoid the plague by observing the Passover. On the evening of January 14, God killed a year-old unblemished lamb or goat and applied its blood to the doorposts of the Israelites' homes, and the blood became a target, causing the plague of killing the firstborn to pass.

[Exodus 12:12~13] "On that same night I will pass through Egypt and strike down every firstborn--both men and animals--and I will bring judgment on all the gods of Egypt. I am the LORD. The blood will be a sign for you on the houses where you are; and when I see the blood, I will pass over you. No destructive plague will touch you when I strike Egypt.

But there were rules about how to apply the blood of the Passover lamb to the doorposts.

[Exodus 12:21~22] Then Moses summoned all the elders of Israel and said to them, "Go at once and select the animals for your families and slaughter the Passover lamb. Take a bunch of hyssop, dip it into the blood in the basin and put some of the blood on the top and on both sides of the doorframe. Not one of you shall go out the door of his house until morning.

In order to apply the blood of the Passover lamb to the threshold, it had to be soaked in hyssop and applied, which was a sign of what Yehoshua would do.

[John 19:29~30] Later, knowing that all was now completed, and so that the Scripture would be fulfilled, Jesus said, "I am thirsty." A jar of wine vinegar was there, so they soaked a sponge in it, put the sponge on a stalk of the hyssop plant, and lifted it to Jesus' lips. When he had received the drink, Jesus said, "It is finished." With that, he bowed his head and gave up his spirit.

Joshua said he was thirsty to fulfill the prophecy of the Passover lamb foretold in the Bible, so the soldiers dipped a sponge tied with hyssop in wine and put it in Jesus' mouth, and Jesus said, "It is finished." And when they tried to break his legs after his death, they saw that he was dead and did not break his legs, which was the fulfillment of the prophecy not to break the bones of the Passover lamb.

[John 19:33~36] But when they came to Jesus and found that he was already dead, they did not break his legs....(36) These things happened so that the scripture would be fulfilled: "Not one of his bones will be broken," 

[Numbers 9:12] They must not leave any of it till morning or break any of its bones. When they celebrate the Passover, they must follow all the regulations.

Therefore, when Yehoshua says "it is finished", it is a prophecy about the Passover lamb. Yehoshua died on the cross at 3pm on January 14th of the Jewish calendar, the day the Passover lamb was sacrificed, which you can read more about at the link below.

http://godnara.egreef.kr/bbs/board.php?bo_table=cs_notice_eng&wr_id=184&page=2