According to People, the boyfriend of the "Wizards of Waverly Place" star decided to throw Selena a prom when he found out she had never been to one. Gomez missed it due to her immense popularity at the time.
My fiancée had never been to prom before... so I asked her if she wanted to take prom photos together at the mall... she agreed!, Blanco confessed.

Source:
@itsbennyblanco
The actress's fiancé shared photos of the evening on his Instagram. In them, you can see that for the "prom," Selena chose a stunning purple dress, while Blanco wore a classic tuxedo.
The comments under the video are filled with admiration for Benny's gesture:
Singer Katy Perry also commented, writing: "I keep loving them."User Comments"The greenest flag ever raised!!!!! We love you, Benny!!!!", "Are you straight out of a romance novel?", "If my man isn't like Benny Blanco, I don't want him"

Source:
imdb.com