Apple has apparently been fairly aggressive in their attempt to land the service on the Apple TV. The company might also offer HBO Now as a separate app from HBO Go, the HBO on-demand service currently on their set-top box. From The International Business Times:

Apple has been most aggressive in courting HBO in a bid to add the service to Apple TV, sources say. Apple TV already carries HBO Go for current HBO subscribers, but it may add a second app for HBO Now. Apple has spent the past several years negotiating for the rights to offer its own linear TV package; in the meantime, HBO Now is seen as an added service to drive adoption of Apple TV.

HBO announced in late 2014 that they would be launching their own streaming service at some point this year. The company is reportedly working with streaming partner Major League Baseball Advanced Media to have the service up and running in April to coincide with the premiere of the fifth season of Game of Thrones. HBO will reportedly set the price at around $15 per month, roughly on par what you would pay with a cable subscription.

Source: International Business Times