Since there is only one God who is not formed then how can the Jehovah’s Witnesses claim that Jesus is a created god?

You have unintentionally, I suggest, created a straw man argument by overlooking the usage of the word before, as found in the passage you cite from Isaiah 43.

Jehovahs Witnesses do not believe Jesus was a god who was created before Jehovah existed. They believe Jesus was created after Jehovah God already existed. In other words, Jesus Christ was the first of Jehovah's created beings.

The Isaiah passage is therefore not an issue with Jehovahs Witnesses. To them, there were no gods in existence before Jehovah because only Jehovah is eternal. No god can be in existence (or can be "formed," the word used in Isaiah 43) before an eternal being. In fact, Isaiah's use of the word formed implies that gods are products of human craftsmen that are meant to be representations of beings/gods who have no existence. (See Jeremiah 10:8-10.)

In conclusion, to the best of my knowledge, Jehovah's Witnesses do not see a contradiction in Isaiah 43:11, which reads, "Before Me no god was formed, nor will there be one after Me." I also assume that Jehovah's Witnesses, along with other bible-believing denominations, would define a biblical "god" or "gods" as non-existent beings whom idolaters worship in defiant ignorance.

ADDENDUM

And yes, the word god is used in at least one other way in Scripture, and that is as a name for God's nemesis, Satan. In 2 Corinthians 4, Paul refers to him as "the god of this world," though the argument could be made that the god of this world can be summed up in the words "Man is the measure of all things" (Protagoras of Abdera, l.c. 485-415 BCE). In other words, the god of this world involves self worship, and the engines of this worship are the lust of the flesh, the lust of the eyes, and the pride of life (see 1 John 2:16 NIV).

Jesus is not a created god anymore than the men who were called gods in psalm 82:6 by Jehovah were created gods.

The context of Isaiah 43:10 is important in understanding what is being said by God here.

The following is a summary of what Albert Barns says on the subject.

In the previous chapter he had severely rebuked the Jews, as being deaf, and blind, and had showed them that it was on account of their sins that these calamities had come upon them. Yet he now turns and says, that they are the people whom he had redeemed, and whom it was his purpose to deliver, and repeats the solemn assurance that they would be rescued Isaiah 43:1-7. This assurance consists of many items, or considerations, showing that they would be recovered, however far they were driven from their own land. 1 God had formed and redeemed them Isaiah 43:1. It followed from this that a God of covenant faithfulness would be with them in their trials Isaiah 43:2. 2 They had been so precious to him and valuable, that he had given entire nations for their ransom Isaiah 43:3. It followed from this, that he would continue to give more, if necessary, for their ransom Isaiah 43:4. 3 It was rite fixed purpose of God to gather them again, wherever they might be scattered, and they had, therefore, nothing to fear Isaiah 43:5-7. II. God asserts his superiority to all idol-gods. He makes a solemn appeal, as he had done in Isaiah 41, to show that the idols had no power; and refers to all that he had predicted and to its fulfillment in proof that he was the only true God, and had been faithful to his people Isaiah 43:8-13. In doing this, he says: 1 That none of the idols had been able to predict future events Isaiah 43:8-9. 2 That the Jewish people were his witnesses that he was the true God, and the only Saviour Isaiah 43:10-12. 3 That he had existed forever, and that none could thwart his designs Isaiah 43:13. Isaiah 43:10, the first part is explained. Ye are my witnesses - They were his witnesses, because, first, he had given in them predictions of future events which had been literally fulfilled: secondly, by his power of delivering them so often manifested, he had shown that he was a God able to save. Neither of these had been done by the idol-gods (compare Isaiah 44:8). Isaiah 44:8, "Do not tremble and do not be afraid; Have I not long since announced it to you and declared it? And you are My Witnesses, Is there any God besides Me, Or is there any other Rock? I know of none."

Clearly Jehovah God established that no idol-gods had any power and worshipping them was pointless and ignorant.

Speaking with the authority of the Only True God who created everything in heaven and on earth He states that there is no God but Him. He alone is eternally existing.

Eternal existence is what real deity is all about. Since Jehovah’s Witnesses teach that God’s Son is created and thus had a beginning he cannot be deity.

So when the “Word”(Jesus) is called a god in John 1:1 it is not proclaiming him deity. It is saying that the Word was Devine or godlike.

It is the same as when the word god is used in connection with other created beings in the Bible to whom the title god or gods was attached.

A good explanation of this is found here where a key paragraph says

The Scriptures do at times refer to actual persons as gods. However, a careful examination clearly reveals that the term “god” in these instances is not intended to designate these individuals as deities. Rather, in the original languages in which the Bible was written, the term “god” was also used to describe a mighty person or an individual who is divine or closely associated with the Almighty God.

Certainly it is appropriate for the highest ranking created being to be called a mighty one, a god and Devine given his intimate association with the One True God Jehovah.

This does not mean he is equal to the one who created him. Nor does it mean he is an exception to the statement by Jehovah in

Isaiah 43:10

You are my witnesses,” declares, Yes, my servant whom I have chosen, So that you may know and have faith in me And understand that I am the same One. Before me no God was formed, And after me there has been none.