Keith Griffith just published an interview with Fahrenthold where he confirms this.
KG: How did the Trump Foundation first come onto your radar?
DF: So, I’d gotten interested in this subject generally because I was in Iowa and New Hampshire covering the lead-up to the primaries and the caucuses. Trump had done that big fundraiser for veterans in Iowa on the 28th of January, and he said he raised $6 million, including $1 million from himself. Most of the money other people gave went to the Donald J. Trump Foundation.
Let's take a moment to remember the backstory here.
It's late January 2016. Trump had already been feuding with Fox News. As part of a pissing match with Fox and fight with Megyn Kelly, Trump announced he was boycotting the January 28th debate over Kelly's inclusion as moderator - a classic Trump dominance play. In grand style he organized his own counter-programming to the debate: a mega-fundraiser for veteran's causes.
The debate went forward without Trump; Trump held his big fundraiser and claimed to raise $6 million - mostly from wealthy friends he hit up to kick in money. He also pledged $1 million of his own money. It all seemed pretty good for Trump on the night of. Sure, he didn't win Iowa. Fox taunted him with claims he'd lost because he skipped their debate. But soon enough Trump won New Hampshire and he was on his way with a series of primary wins that secured him the nomination.
But not long after the event reporters started asking which charities were getting the money, how much money had been doled out and specifically whether Trump had followed through on his pledge of $1 million. The whole self-inflicted wound was classic Trump - broken promises, refusal to answer questions, trying to skip out on his own pledge for give $1 million. It wasn't until late May that Trump finally came clean under press scrutiny - largely driven by Fahrenthold's reporting - and wrote a personal check to a Marines service organization. By that time Fahrenthold had gotten a pretty good look at Trump's modus operandi and the oddity of the Trump Foundation, a family foundation which gave out money that was almost all from people other than Trump - an extremely uncommon way of operating.
Fahrenthold went to Post editor Marty Baron and said he thought the Foundation could use a closer look. That led to a huge amount of the reporting on Trump's long history of making false promises to give money to charities, lying about giving money to charities, using his family foundation to give a political contribution to Florida Attorney General Pam Bondi and more. In the end it was almost certainly the huge prominence Fahrenthold gained through the Trump Foundation reporting that made a still anonymous tipster come to him with the story of the Access Hollywood 'grab' tape.
It is one of those full circle storylines that define the 2016 election. A huge amount of the damaging stories that eventually came out about Trump grew out of that debate dodge and the charity event he put on to justify his boycott. But it wasn't just bum luck. That happened because in the management of that fundraiser he demonstrated the mix of lying, greed, self-dealing and mismanagement that typify who he is. It's perfect. It all comes together.