Beeswax.io Corp, an ad-tech firm that helps marketers build their own in-house digital ad-buying capabilities, has raised $15 million in a new funding round, the company said, another sign of the growing interest in marketers seizing more control of their advertising functions.

The New York-based company offers what it calls a “bidder-as-a-service” platform—a cloud-based system that lets marketers programmatically bid on digital ads and customize the technology to their needs. That appeals to some marketers more than commonly used external buying platforms because it provides more flexibility, according to the company.

Beeswax has around 75 customers from industries such as consumer goods, business-to-business services and media. Clients include Amazon.com Inc. streaming platform Twitch, location-data company Foursquare Inc. and Vivendi SA’s Dailymotion. Pricing starts at $10,000 per month and rises as clients use more capacity.

Newsletter Sign-up CMO Today CMO Today delivers the most important news of the day for media and marketing professionals. PREVIEW

A 2018 survey from the Association of National Advertisers found 78% of members reported having some form of in-house agency last year, up from 58% five years ago. Just 24% of the 237 respondents said they handled programmatic internally. It was the first time the survey asked that specific question, although in 2013, 7% of respondents had in-house capabilities specifically for “programmatic audience buying for online.” ANA members include Procter & Gamble Co., General Motors Co. and Verizon Communications Inc.

“Many advertisers feel they want more flexibility and don’t want to be tied to one partner,” said Gerhard Louw, head of international media management at Deutsche Telekom AG , which isn’t a Beeswax client. In 2017, the German telecommunications company, which controls T-Mobile US Inc., embarked on an internal initiative called “Project Tetris” to realign how it works with its agencies, ad-tech and data suppliers and take more control of its media outlay. Part of the shake-up was designed to bring more expertise in-house.

Beeswax’s latest round, at a $77 million pre-money valuation, was led by venture-capital firms RRE Ventures, Foundry Group and Amasia. Beeswax last raised $11 million, at a $24 million pre-money valuation, in 2016. Founded in 2014, the startup has now raised $28 million in funding and has 65 employees.

The company plans to use the investment to double head count this year in the U.S. and Europe and potentially hire its first employees in the Asia-Pacific region, Chief Executive Ari Paparo said. Beeswax will also invest more in its products, including in newer growth areas such as advertising via web-connected TVs.

Beeswax anticipated that marketers would look to take more of their programmatic capabilities in-house, said Mr. Paparo. Some other ad-tech companies that enable ad buying are more tailored to the agencies that work for marketers, he said.

While the new funding round is relatively modest in size, it comes as investor interest in ad-tech businesses wanes. The sector is undergoing upheaval and consolidation in a digital advertising market dominated by Alphabet Inc.’s Google and Facebook Inc.

Among the bright spots are companies that own in-housing capabilities and those that operate on the software fee model, rather than charging a percentage of ad spend.

Late last year, for example, S4 Capital PLC, the marketing services group led by former WPP PLC CEO Martin Sorrell, acquired MightyHive, a company that specializes in helping brands build out their digital ad-buying capabilities, for $150 million. Earlier this month, Inside Ideas Group Ltd., another firm that specializes in helping brands with their internal marketing capabilities, sold a majority stake to “brand tech” firm You & Mr Jones, which is also an investor in Beeswax and participated in this funding round.

Write to Lara O’Reilly at lara.oreilly@wsj.com