That statement was a lie. In the run-up to the invasion, one year to the day after the September 11 attacks, Trump appeared as a guest on Howard Stern's radio show. In a conversation about terrorism, Stern asked Trump, “Are you for invading Iraq?”
“Yeah, I guess so,” Trump replied. “I wish the first time it was done correctly.”
Here is the audio evidence, which was originally resurfaced by BuzzFeed, showing that this actually happened. The relevant exchange occurs at 3:35.