When an online link accidentally went live in mid-August, a week earlier than Chase planned, people pounced: Hundreds of applications poured in for a card Chase had not yet announced.

“It significantly exceeded our expectations,” Amy Bonitatibus, a spokeswoman for Chase, said of the “tens of thousands” of cards the bank has issued so far. It has been a particular hit with millennials, who make up the majority of the cardholders — a noteworthy detail, given that many millennials have been turning thumbs-down on credit cards.

The card’s rich sign-up bonus caught the eye of Amber Cooney, 29, who works for a nonprofit lender in Austin and is saving for a honeymoon next year in Italy and Croatia. She considers herself a casual credit card user, but she started paying attention to points and rewards a few years ago when she realized that her spending patterns could net her free vacations if she planned carefully.

“Rewards cards made travel a reality for me,” she said. “I went from seeing Niagara Falls and calling that my international travel to visiting four different countries.” Her points have paid for airfare to Argentina and hotels in Paris; she plans to use her Chase card on a trip to Japan this year.

Ben Schlappig, 26, a travel blogger who lives the life of an upscale-travel nomad — he has no fixed address, preferring to live in hotel rooms — plans to shift most of his spending to his Chase Sapphire Reserve. The card offers triple points on dining and travel spending, the two categories that consume nearly all of Mr. Schlappig’s budget.

Chase defines “travel” fairly flexibly, and includes services like Airbnb and Uber. Sapphire Reserve customers have spent $1.5 million already with those two companies, earning 4.5 million points, Ms. Bonitatibus said.