When protagonist Billy Hope visits a children’s group home in director Antoine Fuqua’s latest feature film, “Southpaw,” an infectious melody plays. It’s nothing like the movie’s adrenalin-pumping rap and hip-hop hit singles. But for singer-songwriter Cathy Heller, this is the sound of her success. Literally.

“You need a lot of magic when it comes to combining the song with the visuals,” says music supervisor John Houlihan, who suggested Heller’s tune to Eminem, the “Southpaw” soundtrack’s executive producer. “It has to work magically in a scene but you also need luck to get through a difficult committee process.”

Unlike Eminem’s explicit “Phenomenal” and “Kings Never Die,” and Slaughterhouse’s “R.N.S.,” Heller’s pop wonder, “Turn the Sunshine On,” is not marketed on the edgy “Southpaw” soundtrack. Nor was “Southpaw” among her most lucrative synchs. By sometimes accepting less-prominent placements, however, Heller positions herself for future five- or six-figure synchs.

“It’s a relationship business and it is just one of many uses that I will have with her over the next 10 years,” says Houlihan, who first licensed Heller’s music in 2009.

At a time when even successful indie artists struggle to make a living from shows and record sales, Heller has managed to crack the licensing code. Synchs pay indie artists approximately $3,000 to $15,000 per TV use and $20,000 to $200,000 per ad, plus royalties. Heller’s secret is to deliver something as refreshing as the “mochalicious Oreo frappe” in the McDonald’s ad currently featuring her recording, “Let’s Get Our Summer On.” In a second McDonald’s summer commercial and another for Walmart, Heller’s placements are pure pop with cheerful titles, “We’re Good Together” and “Let Your Colors Shine.”

“The creative called for uplifting, warm music with a lyrical call-out about colors,” says music supervisor Jeff Rutkowski. “Cathy played me ‘Let Your Colors Shine’ and I knew it would be a hit with the client.”

Since Heller owns her masters and publishing rights, she can readily repurpose material. She previously licensed “We’re Good Together” to Hasbro. And prior to “Southpaw,” both NBC and American Airlines created content with “Turn the Sunshine On.”

“You want songs that people can listen to all over the country, and all over the world, for that matter, and feel like it’s familiar, like it’s their favorite song,” Heller says.

It’s a tall order, to be sure, but that can-do spirit and girl-next-door vibe has made a mint for this married 36-year-old mother of two, with 20 licenses just in 2015. But Heller’s DIY formula hasn’t come easy. Eight years ago, she earned her cred cold calling. By 2011, she was offering in-office concerts to music supervisors, sometimes bearing coffee.

“She is just a very positive personality, just like her music,” Houlihan says. Over the past few months, music labels have brought Heller inhouse to consult. At UCLA, ASCAP’s I Create Music Expo and other conferences, she guest lectures. She also leads intimate master classes in-person and by video conference for 15 artists max at $300 a seat.

Her philosophy is simple. “It matters if you care about people,” Heller says. “It matters if you’re genuine and if your music is genuine. It matters if you’re having fun, because if you are, people know it and it comes through in your music and keeps you working hard.”

Under her own licensing and publishing company, Catch the Moon Music, launched in February 2014, Heller represents other indie artists, including the Highfields. This year, she’s netted commissions of 30%-50% on more than a dozen placements for other indie artists in Citibank, T-Mobile, Hefty, Supercuts and other national ads.

Her secret? “Eighty percent is showing up,” Heller says. “Be present, be genuine, be persistent, be kind, be available, be confident. And 20% is having talent and the ‘right’ songs when those doors open to you.”