Mia Hamm played her first game for the U.S. Women's National Soccer Team (USWNT) in 1987, when she was just 15 year old. She led the team to two World Cup victories and two Olympic gold medals and held the record for most international goals scored until 2013, reaching a level of visibility that few female athletes can claim. She's also been a leader in the team's fight for equal pay. After winning the 1999 World Cup, Hamm and the rest of the team threatened to boycott the 2000 Olympics and successfully bargained for a five-year contract that guaranteed members of the UWSNT provisions that included a $2,000 appearance fee for each of the 27 preparatory matches leading up to the games, which at the time was equal to the men's team's appearance fee. But nearly 20 years later, the soccer pay gap has expanded. In March, 28 members of the USWNT filed a lawsuit against the U.S. Soccer Federation (USSF) for gender discrimination and unequal pay. The lawsuit claims that if the men's and women's teams won each of the 20 non-tournament games they are contractually required to play, women's team players would be paid roughly 38% of what the men would be paid. And when the 2019 FIFA Women's World Cup began, it set off a new wave of conversation on the issue of equal pay. "What's happening right now is not just happening in soccer but across the country," Hamm told CNBC Make It at an event sponsored by Johnnie Walker and nonprofit Ladies Get Paid. She shared what she's learned from being part of a decades-long fight for pay equality, and what women and businesses can learn from the fight being waged by the USWNT.

Mia Hamm of the USA gets away from Hege Riise of Norway in the Womens Football Final at the Sydney Football Stadium on Day 13 of the Sydney 2000 Olympic Games in Sydney, Australia. Shaun Botterill /Allsport | Getty Images

Demonstrate value

If soccer or any other industry is interested in closing the gap between men and women, Hamm says, they need to be ready to make an investment. "You invest where you see value," says Hamm. "The fact that people didn't see women as valuable is a mistake and also a detriment to society." During the USWNT's negotiation in 2000, Hamm says she learned that preparation is essential, but so is unity. "That was important for us. We had a clear message of what we were advocating for and everyone felt supported by one another." But ultimately, to advance your cause, you have to demonstrate undeniable value. "We were saying, 'Hey, it's worth it for us not to go the Olympics, and if you want to go find 20 other players, go ahead,'" she says. "Similar to the Women's National Team now, [in 2000] we had the numbers. We had just won a World Cup and they couldn't say people won't come watch us play."

Mia Hamm #9 of the United States signs autographs for fans following an international friendly match against Argentina on April 26, 1998 at Spartan Stadium in San Jose, California. David Madison/Getty Images

Invest in women

Hamm stresses that investing in women isn't just a social stance, it's a smart business move. Hamm links FIFA's increased investment in women's soccer over the past several years to the sport's increased TV viewership. "It just makes good business sense," she says. "We've been screaming this for so long." The biggest market the USWNT reaches is families with children. Investing in developing this fan-base could build a generation of life-long supporters, fans and consumers. Hamm also points to the large number of brands that have signed on in support of the U.S. National Soccer program, in large part, she says, because of the strength of the USWNT. The USSF boasts an impressive list of corporate sponsors and partners including Volkswagen, Nike, AT&T, Johnson & Johnson, Anheuser-Busch, Powerade, Coca-Cola, Secret, Continental Tire, TAG Heuer, Deloitte and Visa.

As the team's popularity has grown, so has the eagerness of brands to be associated with the USWNT and their message. In April, on Equal Pay Day, LUNA Bar announced that it would pay the 23 members of the USWNT roster $31,250 in order to cover the disparity between the bonuses paid to the women and men's teams. On July 9th, two days after the USWNT won the 2019 World Cup, insurance company Allstate announced a multi-year partnership with the team to become the group's presenting sponsor of the team's victory tour. "The U.S. women's national team gives all these sponsors and U.S. Soccer immediate credibility," says Hamm. "We're hearing actual comments that are like, 'We want to be attached to the U.S. Women's National Team, so we're a part of U.S. Soccer."

Fatma Samoura, FIFA Secretary General Stuart Franklin - FIFA/FIFA via Getty Images

Leadership is key