When Destin Bell got the idea for his running app Card.io, he was "broke as a joke," he said on Friday's episode of ABC's "Shark Tank."

To get the idea off the ground, Bell took his idea to game developer Niantic, which created Pokémon Go and sponsors a Black Developers Initiative that financially supports Black game developers. Niantic initially rejected Bell's pitch, so he emailed the company's CEO once per week for three months — ultimately securing $150,000 in startup funds, Bell said.

"That's brilliant, and I hate you, because now everybody out there is going to send us 600 emails a week," investor judge Mark Cuban told Bell in response. (A Niantic spokesperson confirmed Bell's account to CNBC Make It.)

Bell, 26, went on "Shark Tank" to ask the show's investors for more funding. His Austin-based app, Card.io, is a social game where users compete for turf in their city by walking, running or biking. The app is free, with individual and group subscription tiers that remove ads and offer social media-type functions for members.

Card.io brought in $4,000 in revenue, with customers in 70 different countries, the month before the episode was filmed, said Bell. He asked the show's panel for $150,000 — to spend on product and user growth, and Android app development, he now tells Make It — in exchange for a 5% equity stake in his company, which was founded in 2022.

Bell is a longtime "Shark Tank" viewer, he added: His mom initially encouraged him to start watching the show when he was 10, to help fix his lisp and stutter. "I told my mom, 'We're going to be on this show one day, Mom, and I'm going to bring you,'" he said.

'I got to be part of this'

A few of the judges quickly dropped out. Lori Greiner and Mark Cuban both said they probably couldn't help the business grow much, and Kevin O'Leary said he didn't have enough data to justify the investment.

That left Daymond John and guest investor Rashaun Williams, a venture capitalist and minority owner of the NFL's Atlanta Falcons. The pair teamed up to offer Bell $150,000, in exchange for 15% of his company. Bell countered with 10%, which the investors rejected.

Then Bell asked a question: "Can I go talk to my mom?" Bell's mom came on stage and encouraged him to accept the offer on the table. He agreed, and the deal was finalized after the episode's taping, Bell now says.

On the show, Williams and John said they offered Bell a deal because they appreciated his determination and grit.

"I got to be part of this, and I don't care how I'm part of this," said John, the founder and longtime CEO of apparel company FUBU. "I love your energy. I did the same thing. I just did it with a phone. Forty years ago, I made 50 calls a day for six months, until people picked up."

How to write a great cold email

Cold pitches really can work: Cuban, for example, purchased online pharmacy Cost Plus Drugs after responding to a shot-in-the-dark email from its founder, Alex Oshmyansky. A well-written cold email requires research to find the right recipient, establishing a connection with that person, frankness and patience, experts say.

"Connections are key," Monster career expert Vicki Salemi told Make It last year. "Leverage the subject line in the email. Something like, "Hello from an [insert alma mater name] alum" or "Networking via [insert mutual contact's name]" or the [professional industry organization]."

Then, get to the point: Ask directly for what you want, describe what makes you stand out and keep your email short. Once you've checked it over for spelling errors and sent it, try hard to be patient.

"It may take a day or two," said Salemi. "It may take a week or two and that's OK, that's normal and similar to the job search."

Disclosure: CNBC owns the exclusive off-network cable rights to "Shark Tank."

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