Speaking with Zane Lowe for Apple Music, Miley Cyrus talked about how that collaboration came about:

Me and Billy [Idol] have known each other—, I think the first time I ever did anything for Billy Idol was in like 2013. I actually think that’s when I just dyed my hair platinum and shaved my head, I had short little spiky hair and I looked like Billy Idol […] even more Billy vibes, I was pretty androgynous you know and I like really used him as a inspiration for my kind of like—, the transformation that I had, that was a big part of that for me, his his music. Because the way that he kind of married rebellion, but also his music where they had incredible hooks and he showed me that I could have balance, that I could make music that I and other people love. Sometimes I’ve lost that and I’ve found it again where it’s like: “I want to make music for me”, but it’s like part of music is sharing, you know, it’s not hoarding these songs it’s sharing. “Eyes Without a Face” actually followed me around for like a year, anytime I would go anywhere, it would be playing. It was really trippy and I really started to think that life is just all— I mean, it’s the Truman concept, I’m like: “Who the hell keeps keeping playing “Eyes Without a Face”?” I’d go to the grocery, I go to a restaurant, anywhere I went, it would be playing and then I went to the studio with what I had said, I had just been going through like all different relationships and just figuring it out and I was telling him that I felt like I thought that Billy Idol music videos are like porn. I think the old billy idol music videos are so sexy. I’m like: “Yo, that is my porn preference, is Billy Idol music videos.”, and so I’m like: “Oh man, we got to write a song about that.”, and he’s like: “Let’s just write a song with Billy Idol, let’s just get the real deal here”. So we we got Billy to come and write a song and actually […] I think when I fell off and was drinking, there was a moment where I told Billy Idol, that his music videos are porn […] over dinner, this is a regular polite dinner conversation.
The chorus, where it says: “Under the disco ball”, the disco ball I have is in my house in Nashville, Tennessee and me and watt were separate for New Year’s. I was in Tennessee and he was in LA and we were FaceTiming under the disco ball saying: “We’re making the best record of all time. 2020 is our year. We’re gonna make this album.” We were so stoked and we were under the disco ball.
[…] Then my other favorite story is uh Ryan Tedder wrote this with us and we’re all in the studio and we’re like: “Hey Billy, could we just get a couple ad-libs?”, and he goes: “Yeah, yeah!”, and we’re like: “Please say someone was recording this!”, and we got it and that was his real response to it, was like an ad-lib. You can’t even say “Yeah” without going “Yeah!”