By Anya George Tharakan

(Reuters) - MediaBrix, a fast-growing startup whose software delivers "moment-based" mobile advertising, is going after one of the most difficult demographics in the ad business - teens.

MediaBrix gives advertisers a chance to target people based on what is happening when they are using an app, as well as on the emotions commonly associated with those circumstances.

For example, a game player who has moved up level may get an extra "life" through an ad delivered by MediaBrix from a company such as chocolate maker Hershey Co.

The combination of moving to the next level, plus getting an extra life, gives the ad greater impact and builds a positive connection between the user and the brand, MediaBrix says.

On the other hand, a player facing the unhappy prospect of having to pay for a premium feature may be targeted with a video ad from, say, Ford Motor Co, that will reward the player with the feature in return for watching the ad.

Again, a positive relationship is established, the New York-based company says.

The formula is used in more than 1,000 messaging, fitness, sports, lifestyle and other apps.

The company also does surveys to ask app users what they are feeling while they are using the app. So, for example, if most people using a fitness app say they are happy with a workout, MediaBrix knows it is probably a good time to target an ad.

"From day one, MediaBrix has focused on putting the human experience at the center of mobile advertising," co-founder and Chief Executive Ari Brandt said in an interview.

"When brands put a user's needs before their own, they have a receptive user who appreciates the brands – a win-win situation," Brandt said. "That's what we cultivate."

It's a strategy the company hopes will allow it to generate $100 million in revenue in 2018. MediaBrix said it is operating at a current run rate of $30-$35 million in revenue for 2016.

Teenagers are a particular target.

"Teens are notoriously anti-advertising and finicky about how brands participate in their space, so our approach ... is perfectly suited for and embraced by the mobile-first generation," Brandt said.

Apart from Hershey and Ford, clients include Starbucks Corp, Nike Inc and AT&T Inc.

The company, whose investors include Edison Partners, Revel Partners and Horizon Technology, has raised a total of $18 million since launching in 2011.

The potential is huge. The worldwide internet-advertising market spending is set to grow 51.7 percent to $108.9 billion in 2016, according to research firm eMarketer.

MediaBrix competes with Kiip, another "moments-based rewards" platform.

(This version of the story corrects paragraph 11 after MediaBrix says it is operating at a current run rate of $30-$35 million in revenue for 2016, not expecting in revenue)

(Reporting by Anya George Tharakan in Bengaluru; Editing by Ted Kerr)