Will Ferrell’s Son Magnus, 20, Attends You’re Cordially Invited Premiere, Reveals How Dad Got His Song Into Movie (Exclusive)

Will Ferrell and Reese Witherspoon’s latest film, You’re Cordially Invited, is a family affair on and off screen!

At the New York City premiere of the new romantic comedy film on Tuesday, Jan. 28, the actor, 57, and actress, 48, were joined by the cast and crew, including Ferrell’s eldest son, Magnus. (The Anchorman actor shares Magnus, 20, Mattias, 18, and Axel, 15, with wife Viveca Paulin.)

Magnus tells PEOPLE what it was like to get his song, “Slow Down,” featured in the upcoming movie. He says that while his dad was in the middle of filming, he and his roommate at USC “were kind of in a rut, and wanted to make something weird and out-of-the-box.”

The pair then experimented and made a “really weird-sounding, super-fast, kind of Latin” song, he tells PEOPLE, adding that they shelved it until Magnus decided to send it to his dad.

“Then he was like, ‘Hey, we’re filming but they don’t have this song yet in this party scene. I showed them yours and they’d love to use it,’ ” Magnus recalls. “I was like, ‘Yeah, please. Let’s do it!’ Just super fortunate to actually be a part of something.”

The young musician was recently signed to Republic Records and tells PEOPLE the Tuesday event is “the first major thing [he’s] been able to do.” He adds that being signed has been “special” and “surreal,” and he’s “just excited to get going.”

“I’ve got such an amazing support system and people that really believe in me,” he says. “I’m just so lucky that I found such a good group of people.”

The upcoming film stars Will as Jim, the father of the bride, portrayed by Geraldine Viswanathan, and Witherspoon as Margot, the sister of a different bride, who Meredith Hagner portrays.

Although Margot and Jim don’t know each other, their worlds soon collide when they find out their respective wedding parties have been accidentally double-booked at the same remote venue. Quickly, chaos ensues as they’re forced to battle it out for the best picture-perfect ceremony.

Jimmy Tatro, Leanne Morgan, Ramona Young, Jack McBrayer, Celia Weston, Rory Scovel, Stony Blyden and Keyla Monterroso Mejia also star in the Nicholas Stoller-directed film.

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You’re Cordially Invited is available for streaming on Prime Video on Thursday, Jan. 30.