The 32-year-old singer took the stage at the MGM Grand Garden Arena in Las Vegas to showcase his song "Twin Flame" from his album "Mainstream Sellout."
Sporting painted nails, ripped jeans and mussed hair with a light magenta hue, MGK started by quietly saying he wrote the song for his "wife."
MGK, real name Colson Baker, and Fox have not publicly shared they are officially married, but the tune was clearly for her as she lovingly watched from the crowd.
On Tuesday, Fox reflected on her love's performance on Instagram.
"Baby- so proud of you for giving such a vulnerable and emotional performance. You are magic," she wrote alongside photos from the awards show.
The couple revealed in January they were engaged through intense, heartfelt posts on Instagram.
"Somehow a year and a half later, having walked through hell together, and having laughed more than I ever imagined possible, he asked me to marry him," Fox wrote. "And just as in every lifetime before this one, and as in every lifetime that will follow it, I said yes."
And then, Fox shared, "we drank each other's blood."
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MGK interrupted the romantic rock ballad during his BBMA set to look into the crowd and say, "Happy birthday, my love," before detouring into a few lines of "Born With Horns." Fox turned 36 on Monday.
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When he returned to "Twin Flame," he dedicated it to "our unborn child" and scooped up his electric guitar to crank out some serrated riffs. It is unclear what he meant with those words, but he later wrote on Twitter, "broke my heart to sing the end of that song." Neither have addressed it on social media.
MGK and Fox debuted their relationship in the summer of 2020 and ever since have been open about their fierce love for each other.
In an an interview with Glamour UK, published April 26, Fox cleared up what the couple means when they talk about drinking each other's blood.
"Yes, we do consume each other’s blood on occasion for ritual purposes only," Fox told the outlet, noting it's not "goblets" of blood but "just a few drops."
"When I do it, it’s a passage or it is used for a reason. And it is controlled where it’s like, ‘Let’s shed a few drops of blood and each drink it.’ (MGK is) much more haphazard and hectic and chaotic, where he’s willing to just cut his chest open with broken glass and be like, ‘Take my soul.' "
Fox and MGK often describe each other as their "twin flame," hence the name of the song. The "Transformers" actress also said that she believes she manifested her relationship with MGK as a young child.
"He’s literally my exact physical type that I’ve been manifesting since I was 4. I’m also four years older than him," Fox said about her fiancée. "So, I think I made him."
Contributing: Melissa Ruggieri, Hannah Yasharoff
This article originally appeared on USA TODAY: Megan Fox praises MGK for 'vulnerable' Billboard performance