Boingo Wireless Inc. will help Microsoft Corp. promote its new Windows 8 operating platform from now through the end of the year by offering free sponsored Wi-Fi access at subway stations and other high-traffic areas in the San Francisco Bay Area and New York.

The Los Angeles wireless Internet hotspot provider already has a significant presence at transit stops in those two metro areas. A Boingo spokeswoman said that Microsoft was particularly interested in reaching mobile app developers in the Silicon Valley and Silicon Alley tech communities. Windows 8 is set to be available for pre-order on Oct. 21.

The Wi-Fi hotspots are part of Boingo’s Cloud Nine media platform, a global advertising network that enables advertisers to reach target audiences through Wi-Fi sponsorship.

“We do a lot of long-term and short-term sponsorship deals, and work with clients to reach the groups they want to target,” said Katie O’Neill of Boingo. She noted that the Microsoft deal, terms of which were not disclosed, is the fourth such arrangement with a corporate sponsor in three months. “It’s a growing part of our business.”

A Microsoft representative could not be reached for comment on why Los Angeles was not part of the promotion. O’Neill noted that Boingo does not have hotspots at Metro stations here, and its biggest presence in high-traffic areas includes shopping malls, such as Americana at Brand in Glendale.

Boingo’s stock closed down 15 cents, or about 2 percent, to $7.60 on the Nasdaq on Tuesday.