The actress/singer became engaged to musician Koma last month (May 2019), but Duff reveals she wasn't in the best of moods when he suggested they take a stroll to a park near their Los Angeles home, because the new mom was so exhausted.
She recalled, "I came home from work on a random Tuesday, and he's like, 'Let's go for a walk. Let's go check out this park....' "
"I was like, 'I'm so tired... I just wanna eat dinner in bed and watch Game of Thrones'."
Koma insisted on enjoying a little alone time with Duff, who only agreed to go if she was able to change into her sweatpants.
It was only after arriving at the "gorgeous park" that her man revealed his true intentions as he gave her a book detailing their love story.
"He presents me with this book that he said he's been working on for a while... and he had this amazing guy do all these illustrations and was like, 'We're gonna forget these (memories) along the way, so we need to constantly fill this out so our kids can know our story'," Duff told U.S. daytime show "The Talk". "And when I got to the end, there was a little part that I peeled open and my ring was inside."
The rush of emotions they both experienced at that moment made Koma forget his pre-prepared speech for his proposal.
"He got down on one knee and was like, 'I forgot everything I was supposed to say but you're my best friend'," Duff gushed. "It was so, so sweet."
The marriage will be Duff's second - she and ex-husband Mike Comrie share seven-year-old son Luca, while the star welcomed daughter Banks with her new fiance in October (2018).