"We were really, really drunk, and we went back to his house," she tells Radio.com. "I laid on top of his piano and he played this amazing piece of music and we started writing 'Kingdom.' It just kind of like sat as this weird, drunken voice note on our phones for a while."

It wasn't until Lorde-who Charli calls by her given name Ella-called her up and asked her to lend a song to the soundtrack, which she not only appears on but also curated, that Charli figured out what to do with it.