According to the Blasphemy Challenge website, you’re supposed to say "I deny the Holy Spirit" and that is supposed to be the unforgivable sin mentioned in Mark 3:28-29 “Truly, I say to you, all sins will be forgiven the children of man, and whatever blasphemies they utter, but whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin"”
Blaspheming against the Holy Spirit is not insulting Him, or saying that you don’t believe that He exists. To understand what it is to blaspheme against the Holy Spirit, you have to understand the work of the Holy Spirit. His work is basically to help us understand how we get forgiven, namely, by trusting in Jesus:
John 16:8-9 (Jesus speaking about the Holy Spirit) “And when he comes, he will convict the world concerning sin and righteousness and judgment: concerning sin, because they do not believe in me;”
So to blaspheme the Holy Spirit requires that one reject Jesus continually. This is unforgivable, because forgiveness comes through faith in Jesus.
God will still forgive you if you want his forgiveness.