Kristen Bell helps teen with 'Frozen'-themed promposal on 'Jimmy Kimmel Live!'

It was the promposal to end all promposals.

While filling in as host on Jimmy Kimmel Live! Thursday, Kristen Bell helped a teen ask his classmate to prom by singing an altered version of the hit Frozen song, 'Do You Want to Build a Snowman?'

"Do you want to be his prom date and go with Michael to the prom?" Bell sang for Sarah, who had no idea that she would be center stage at the taping she and her family attended.

Sarah had been sent out to Hollywood Boulevard in Los Angeles wearing an 'Anna' costume, tasked with finding a masked 'Kristoff' on the street.

"Now to make things more dramatic, Sarah and Michael have never been on a date before," Bell said while Sarah was out searching. "Sarah has no idea Michael is asking her to prom. This could go terribly wrong, but you've got to admit it would make great TV, right?"

Read: Emma Stone Responds To Teen Who Asked Her To Prom In Epic Video

The teen found the character and when she brought him back to the studio, she was surprised to find her classmate Michael in the suit.

"Sarah, there's something I wanted to ask you, but I didn't know the words, so Kristen Bell is going to help me," Michael said.

Stunned, Sarah turned to face Bell, who said: "Sarah, I want you to think about this. Don't answer until I'm done singing."

Read: Girl With Down Syndrome Will Take Helicopter To Dance After Doritos Promposal

To the tune of 'Do You Want to Build a Snowman?' Bell sang: "Do you want to be his prom date and go with Michael to the prom? Michael wants so much for you to go, because if you say no, he'll have to take his mom. He knows you're just school buddies, but what the f***, he thinks you're really cute.

"He's asking you to be his prom date, in front of millions of people," Bell continued. "If you say no this will be awkward..."

Sarah interrupted at that point, giggling as she grabbed the mic and said, "Yes!"

Bell set the pair up with a limousine on the big day, as weel as a glam session to help Sarah get ready and a tuxedo fitting for Michael.

"And a case of beer! I'm just kidding about the beer," Bell quipped. "Congratulations!"

Watch: College Students Create 'Frozen'-Themed Prosthetic Arm for 9-Year-Old Girl

PHOTOS: Kristen Bell through the years