Growing up, former Duke basketball star RJ Barrett envisioned his NBA draft day — particularly walking across the stage and greeting the league commissioner, Adam Silver.

“Shaking the commissioner’s hand has always been a big thing for me,” Barrett, 19, tells The Post. “When you’re young and you watch the draft every year, you’re like, ‘That’s going to be me one day.’ ”

But until recently, the Toronto native hadn’t thought about what he would wear for the most important handshake of his young life.

Over the past few weeks, he’s worked quickly to figure out his approach for Thursday night’s draft, which is being held at the Barclays Center. Like his college hoops career, he wanted it to be splashy, sharp and a “one and done” — the industry term for players who attend college for a single year before becoming NBA eligible.

“The suit is going to be put in plastic and preserved [after the draft]. Maybe I will go into my closet and look at it a bit. Maybe I’ll frame it. Or at least the jacket,” explains the lean, 6-foot-7 small forward, who is expected to be a Top 3 pick.

Barrett, who describes his everyday style as “sweats,” will be sporting a bold pink suit with a black dress shirt, a black tie, gray Louboutin loafers trimmed with studs that retail for $995 and his brand-new Cartier watch.

“It’s extravagant. I’ve always been a reserved kind of person, so the thought was, ‘I want this to be memorable.’ It’s a night that I will never forget, so I want my suit to replicate that.”

NBA draft suits have become an integral part of the annual hoops selection showcase. Once a parade of clownish Steve Harvey-esque ensembles, the event — like the league’s current superstars — has steadily become more style-centric. Today, it’s a sartorial rite of passage, where young soon-to-be millionaires show off their fashion chops with natty tailored suits, peacock prints and audacious footwear.

Many prospects work with suitmakers, stylists or brands such as J.C. Penney or Express to create their look for the big day. Barrett, however, has turned his draft suit into a business deal. He signed a multiyear contract with Canadian made-to-measure suit brand Indochino and will become the company’s first signature athlete. Thursday night will be his inaugural appearance as brand ambassador.

“I’m thankful and honored, because look at how many people are in the draft, and Indochino chose me,” says Barrett, who hasn’t spoken to fellow Duke teammates and lottery picks Zion Williamson or Cam Reddish about their getups for the big day.

“We did say that at the draft we were going to look good. Everything is going to be a surprise. They don’t know what I’m wearing, I don’t know what they’re wearing.”

And although he has numerous experts in his life to consult — his godfather is former NBA superstar Steve Nash — he turned to the internet for sartorial inspiration.

Barrett studied previous draft classes to rule out certain looks that had been done over and over — including a burgundy tux that he’d been drawn to.

“It was really nice. But I saw a lot of [other athletes] have worn the same one.”

Then, in his Southern California digs, he pored over swatches before finding his fabric: a saturated, light pink merino wool.

To truly put the Barrett kiss on his suit, the symbol of his motherland — a continuous pattern of the Canadian maple leaf — is sewn into the lining, which he plans to show off when he poses with Silver onstage.

“I want to unbutton my jacket, pop it open, and you can see where I am from. I gotta rep my country everywhere I go. I want to be that guy who comes to mind when you think of Canadian basketball one day. I still have a long way to go, but that’s definitely one of my goals.”

‘It’s a night that I will never forget, so I want my suit to replicate that.’

Also stitched into the interior of the jacket is his nickname, Maple Mamba — a play on Kobe Bryant’s Black Mamba moniker, which Barrett earned during his year at Duke.

“Someone said, ‘You’re from Canada. You have Kobe’s mentality. You have the maple leaf, and you get Maple Mamba.’ ”

Barrett, who “loves” his nickname, says that his suit “shows loyalty” to his origins.

“Mamba started while I was at Duke, and I have my Canadian flag, so it’s the places where I come from, the places that built me. Whatever team I go to, that will be part of my journey as well.”

Barrett hasn’t been shy about where he wants to land: He hopes the Knicks, who have the third pick, call his name. The phenom’s parents both attended St. John’s University in Queens, and his mother is from Brooklyn, where he spent summers as a child. His late maternal grandfather — a rabid Knicks fan — once predicted his talented grandson would play for them.

“Hopefully I get drafted and just have to come over the bridge,” he says.

While he can’t be sure he’ll be calling Madison Square Garden home, he is willing to guarantee one thing.

“I will cry for sure,” says Barrett.

Onstage?

“Oh, hell no. I will cry in the back, though. I will need a minute to myself.”

Two days before the draft, family and friends gathered in Barrett’s East Side hotel room to watch him try on two custom suits from Indochino. He put on a black chalk-stripe suit with a double-breasted vest that he plans to wear for the Friday press conference with his new team.

And, of course, he tried on “the one”: Barrett slipped into the pink trousers, then shimmied and smiled like a bride finding her wedding gown.

“I feel like a million bucks. I can’t wait to show the world. I feel like a grown-up. This is a big stage in my life,” he says. “I’m not just a kid going to school anymore.”