No one was more surprised by the success of Harry Styles’ “Adore You” than Amy Allen, the woman who co-wrote it. “When a song of mine comes out, I always tell my managers [Jonathan Eshak at Mick Management and Gabz Landman]: ‘Don’t say anything unless something remarkable happens,’” says the 28-year-old singer-songwriter, musician and producer. “I don’t even check Mediabase for the radio stats — I try to live under a rock. Ever since I was little, I’ve obsessed over things. My mother always told me: ‘Once it’s out of your hands and into the universe then you have to let it go.’ ”

But last week, something remarkable happened. “It just crossed into the top 10 at radio, so one of my managers had my approval to text me,” she says. “I was like, ‘Oh my God!’ I had no idea. I was so excited. Deep down I’ve always been hungry to put out songs with a universal message that the world can sing back — and I guess the easiest way to sum that up is a top radio song.”

“Adore You,” released by Columbia Records, has 82,727 total spins to date per Mediabase and recently hit No. 1 on SiriusXM’s Hits 1. According to BuzzAngle Music, the song moved 66,988 song project units; Styles’ album “The Fine Line,” is about to cross 2 million album equivalent units.

But “Adore You” doesn’t sound like anything else on the airwaves in 2020. “There’s a guitar solo, for one, which is very rare,” notes Allen, who graduated from the Berklee College of Music. “It has a feel that is reminiscent of the ’70s. I grew up on that classic soft rock — Fleetwood Mac and the Stones’ ‘Wild Horses.’ ” The single stands out lyrically, too. “It’s totally a love song and a lot of the music on the radio is pretty dark and serious right now,” she adds.

Ironically, dark songs are her forte — she co-wrote “Graveyard” for Halsey as well as “Without Me,” the latter which went to No. 1 at pop radio after spending a record-breaking 29 weeks in the top 10. “I don’t know what’s wrong with me: I’m a happy person, but I can’t write happy songs,” says Allen. “‘Adore You’ was my first feel-good song, so I’m psyched about that.”

Another highlight of the song’s journey from pen to life: “Watching Harry,” she says, referring to Style’s one-night-only album release show at The Forum. “I was really nervous because the album had only been out for a couple days and I wasn’t sure if anybody would know that song.”

To her surprise, most of the 20,000 in attendance sang along to “Adore You.” “Harry is a phenomenal performer, the band sounded amazing and he has the most loyal fans,” says Allen. “My friends Tom [Hull] and Tyler [Johnson] — the other writers on the song — were also in the crowd so it was just a special moment. I haven’t seen any songs I’ve been a part of played live before. … It was beautiful and really cool.”

But not quite as cool as seeing Lizzo sing “Adore You” — complete with a cameo from Sasha Flute — on BBC Radio 1. “I idolize Lizzo,” says Allen. “It really just goes to show that the right song can be performed by many different people. Everybody at the end of the day has the same emotions and feels like they would do anything because they loved somebody so much.”

As it turns out, Allen expected to see Lizzo’s cover version a couple weeks ago — as a duet with Styles over Super Bowl weekend. “I was with Harry on his birthday and the night before he had been in Miami,” she recalls. “Lizzo was going to do it with him on stage and then the show got rained out. So I was, like, ‘Aw, man! I would’ve loved to have seen the video of you guys doing it together.’ And then I didn’t hear anything else about it.” (Worth noting: Styles joined Lizzo in Miami for her own hit “Juice” on June 30.)

Allen’s managers followed her instructions for a gag order, but then something remarkable happened — again.

Watch Lizzo’s take on “Adore You” below: