The City of Boston is offering free two-hour parking between now and Jan. 2.

Residents and visitors must use the ParkBoston app, which is free to download for iPhone and Android users, and enter the promo code “617617,’’ the mayor’s office said. Each ParkBoston account will be allowed to use the code once.

About 25,000 people have downloaded the app since the free-parking promotion started Nov. 27, said Gina Fiandaca, commissioner of the city’s transportation department. But only about 450 users have entered the promo code, she said.

The promotion is a method to reward people to come into the city on a weeknight or weekday because they can choose when to use the discount code, she said. It’s also a way to help push Mayor Martin J. Walsh’s “#5onMain’’ challenge to encourage holiday shoppers to buy from local businesses, Fiandaca said.

All 8,000 metered parking spaces around the city are now connected to the ParkBoston app, which allows users to pay for parking without quarters. There is a 15 cent “convenience fee’’ for each transaction, according to the ParkBoston website.

Walsh has emphasized incorporating more technology, and on Tuesday, Alphabet Inc.’s Google said Boston was the “2015 eCity of Massachusetts.’’ The award recognizes the strongest online business community in each state, according to the mayor’s office.

Jessica Geller can be reached at jessica.geller@globe.com. Follow her on Twitter at @jessicageller57.