How to get free fries at Burger King

New York(CNN Business) Burger King is giving away free fries — but there's a catch.

The new perk is available only to members of Burger King's free Royal Perks digital loyalty program. Beginning Monday, customers can add french fries of any size (small, medium or large) to their purchase once a week at no extra cost. The offer is available for the rest of the year.

The promotion is aimed at expanding Burger King's seven-month-old loyalty program, which the company said in a press release is a "major focus" for Burger King as it shifts away from paper coupons. Restaurant Brands International (QSR), the chain's parent company, said in a previous earnings call that it's "pleased with the early results" of sign-ups.

Burger King previously juiced its fledgling rewards program by selling a Whopper for 37 cents in celebration of the sandwich's 64th anniversary. It also gave rewards members early access to its new Ghost Pepper Chicken Nuggets in October 2021.

Ultimately, the goal for Burger King and other brands with loyalty programs is to increase sales and lure in customers, ideally on a daily basis, by giving them reasons to choose them over their competitors. The programs also give restaurant owners access to customers' data and ordering habits that help target diners with customized deals.

McDonald's (MCD), which launched its rewards program a few months before Burger King's, quickly surpassed 20 million members. Wendy's credited its digital loyalty program for its "strong sales" in its most recent earnings call, with executives saying they expect "meaningful growth" in the program this year.