Cyrus, who is engaged to actor Liam Hemsworth, posted the image in a bid to get the 18-year-old singer's attention on behalf of her younger sister, Noah, who is a huge fan, reported Aceshowbiz.
"All I want for my birffffday is a big booty hoe. All noah wants for her BIRFFFDAY is @harry_styles," Cyrus said.
Styles recently split from girlfriend Taylor Swift.