ET's Cassie DiLaura spoke with the couple on the red carpet at the star-studded show, held at the MGM Grand Garden Arena in Las Vegas on Sunday, and MGK dished on his forthcoming performance of "Twin Flame," which he wrote for Fox.
"It's actually really cool because it's a lunar eclipse tonight, it's a full moon," Kelly shared.
"And it's gonna be my birthday tomorrow," Fox chimed in.
"I wrote this song two years ago exactly today," Kelly recalled. "I played it for her, it's all a trip, it's like it's all meant to be."
"Yeah there's like super special secret meaning to the song as well," Fox shared. "So I'm sure I will be [emotional], but I can't cry and f**k up this eyeliner."
However, the actress predicts that she's going to be moved to crying tomorrow when she and her beau celebrate her 36th birthday on May 16.
"We're going away into the desert tomorrow, with no cell phones," said Fox. "It will be an eclipse, so I'm sure I will be crying a lot."
Meanwhile, the couple looked stunning on the red carpet, with Kelly taking his own photos of his fiancée.
"Always get the good angles, very easy with her," he gushed.
Kelly also showed off the $30,000 worth of diamonds on his nails, which he explained was going to a good cause.
"I’m going to take these off and turn them into rings and we're donating 100 percent of the proceeds to a Cleveland charity, so I’ll specify and release exactly which one in a press release with my nails, so we see that my word is bond," he said.
The Billboard Music Awards air Sunday, May 15 at 8 p.m. ET / 5 p.m. PT on NBC. The show will be streamed live on Peacock.