"It's awesome to get to attend their wedding and it was a blast, it was a really good time. And that was like the first place we debuted the song," Smyers told ET at this year's CMT Artists of the Year event at the Schermerhorn Symphony Center in Nashville, Tennessee, on Wednesday. "We had never rehearsed it!"
"We obviously recorded it in the studio, but we never played it live in front of actual human beings," Smyers added. "[But] Justin was like, 'Sing '10,000 hours,' we're like, 'OK, we'll try.'"
"[We were] just making up the words," Mooney joked.
"It was good though, it was really fun and people were out on the dance floor rocking," Smyers added. "It was a good time."
As for how the song all came together in the first place, Mooney explained that their mutual friend and manager, Scooter Braun, managed to connect them, and that Bieber has "been a fan of country for a long time."
"So we've always wanted to do some collaboration with him and we wrote the song and thought it would be perfect and sent it over to him not knowing what would happen," Mooney recalled. "He loved the song, he's in a similar place in his life, just got married and so it was really cool to have that moment all together."
For the music video, Smyers, Mooney and Bieber came together with their real-life lady loves, and the end result was a beautiful, romantic celebration of happiness and adoration.
"The timing of it worked out and we still can't believe that it happened," Mooney added. "I'm waiting to wake up tomorrow and [find out] like, 'That actually never happened to you guys.'"