The cantrip can replace any of the attacks, including the first. It does not need to be the second attack - that wouldn't work because the Haste Attack action will only give a single one. But since it can replace the first, then does it work?
Oh, I get it. I misread the question. I thought you were asking if the bladesinger could make a weapon attack as part of the Haste, plus a cantrip, and, since a cantrip isn't a weapon attack, use both.
I would still rule no. It's part of the Extra Attack feature for the subclass. But I don't think it's game-breaking to rule otherwise.