The 30-year-old actress accepted "Mr. Robot" creator Sam Esmail's proposal in August 2015, reported E! online.
"You'll find out tonight. Yeah, she'll be making it. That's all I'll say!" the bride-to-be said at an event honoring Herrera in New York City.
After wearing Herrera at the New York Botanical Garden's 2015 Conservatory Ball, Rossum had thanked the fashion mogul on Instagram for the glam look.
"Look at how fabulous she is, like a fashion fairy godmother watching over to be sure the fit is perfect! She is kind, elegant, wickedly funny and I'm lucky to call Carolina my friend," she wrote.