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MediaMath Inc., a digital-advertising company, agreed to a 15-year lease at 4 World Trade Center, becoming the lower Manhattan skyscraper’s first non-government tenant.

MediaMath is taking 106,000 square feet (9,800 square meters) and plans to move its more than 300 New York-based employees from three locations in Midtown to the 72-story tower, developer Silverstein Properties Inc. said today in a statement.

The agreement brings occupancy in the 2.3 million-square-foot building, which opened in November, to 53 percent. Demand for offices in lower Manhattan surged in the first half of the year, in part because of leases at the Brookfield Place complex, where Bank of New York Mellon Corp. and Time Inc. took more than a million square feet combined. Downtown’s vacancy rate of 10 percent is now lower than Midtown’s 11 percent, which is a rarity, according to brokerage Cushman & Wakefield Inc.

“MediaMath’s decision to relocate to 4 WTC proves that New Yorkers were right to bet on downtown,” Larry Silverstein, chairman of Silverstein Properties, said in the statement. “The new World Trade Center is emerging as the commercial heart of the city’s hottest neighborhood.”

Previously, the skyscraper’s only two tenants were New York City and the Port Authority of New York and New Jersey, owner of the World Trade Center site. They agreed to rent 600,000 and 520,000 square feet, respectively.

Silverstein last month said he will resume construction on 3 World Trade Center after a financing deal with the Port Authority. The bi-state agency agreed to let the developer use $159 million in insurance proceeds that had previously been held in escrow until the planned 80-story tower was finished.

The nearby 1 World Trade Center, scheduled to open by year’s end, is about 56 percent leased following the addition of three tenants in the past few months, including advertising firm KiDS Creative LLC. The Durst Organization and the Port Authority are developing the skyscraper, which is the Western Hemisphere’s tallest at 1,776 feet (541 meters).