The former Dancing with the Stars partners hit the 24th annual Elton John AIDS Foundation's Oscar viewing party in West Hollywood, just one day after announcing their engagement to PEOPLE exclusively.
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As for how the proposal went down, Herjavec spilled a few details at the viewing party Sunday.
"We went to the house and I had rose petals going up to the pool," he told reporters. "In the pool was a big swan and on the swan was a box. She pulled the swan in, opened the box, and I said, 'Actually that's not for you. That's for me.' "
"And I opened the box and the ring was inside. And I said, 'Will you marry me?' and she started crying and doing crazy sounds," Herjavec continued, laughing. "And then I asked again and she started crying and doing crazy songs. And I asked a third time and I was like, 'OK this is really uncomfortable.' "
According to Herjavec, who said "about a month" of planning went in to the proposal, keeping it under wraps was "unbelievably" difficult: "It was a lot of pressure. I'm not good at keeping secrets!"
Johnson, meanwhile, said she "had no idea" it was coming.
"He was acting a little strange all week, I'm not going to lie," she told reporters with a laugh. "He was just stranger than normal."
The dancing pro, 39, said she is "so happy" and called the proposal "pretty amazing."
"He flew my mom over so that's more than asking [permission]," she said. "He proposed to me privately and then we went back home and there was all my friends and my mom and family there. So we had a celebration straight away that was really special."
While the two haven't started planning the nuptials yet, they're thinking of having a wedding state-side and then throwing a party in Australia, where Johnson was born.
"We might go back to Australia and do a big party there with all my friends and that one will probably be pretty wild, you know Aussies get crazy," she said.
And though they haven't nailed down too many specifics just yet, there's one thing they're already sure about: the song they'll have their first dance to!
"When we were on [DWTS] last year, the week before we got kicked off, they gave us a song. And it was Etta James' 'At Last,' " Herjavec explained.
"It was our next dance to do but we got voted out so we never got to do it. So it will be our first dance as husband and wife I guess!" Johnson added.
"It's just going to be really romantic," she continued. "I'm pretty sure people will be expecting a dance. Considering the way we met, we kind of have to do a dance."
Johnson and her Shark Tank star fiancé, 53, were paired up on Season 20 of the show last year and began dating ten months ago.
"I still can't believe that a year after walking into a dance studio of all places, my life has changed this much," Herjavec, who popped the question with a gorgeous 6.5-carat ring, previously told PEOPLE. "Kym is an incredible woman and I'm just so excited for what the future brings for us."
"I'm just overwhelmed and so happy," Johnson said. "He's a wonderful man and I'm so lucky."
This will be the first marriage for Johnson, who recently launched her 5678 Diet book and 5678 Fitness DVD. Herjavec has three kids with his ex-wife.
The 88th annual Awards, hosted by Chris Rock, were being presented live on ABC from the Dolby Theater in Hollywood, California.