Sitting at a piano, Swift played and sang the tune that she co-wrote under a pseudonym for the crowd.
"I've never played this song live before," she said. "But if you know it, maybe you could sing along, and then I could get to know what that feels like to hear it sung back to me."
The hit song made waves over the summer after Harris tweeted that Swift, who had wanted to keep her identity and involvement under wraps, wrote the lyrics to the song with Rihanna. She is credited with the pseudonym Nils Sjoberg.