Sling TV announced it's adding HBO as an add-on service before the season premiere of "Game of Thrones" on April 12. Sling TV will launch HBO as a $15 add-on in coming days, making the premium cable channel's live programming and video-on-demand library available through the streaming video service. Sling TV, the over-the-top video subsidiary of Douglas County-based Dish Network Corp. , announced Wednesday that HBO will become available before the April 12 season-five premier the popular fantasy series "Game of Thrones" and before the season's debut of "Silicon Valley."