You don’t, at least not using the Directv app. The Directv app does not support casting to a TV. You can use the various network apps, though. To cast from iPhone to TV either requires airplay through an Apple TV or directly connecting iPhone to an hdmi input with an adapter cable.
Award for Community Excellence 2019 Achiever* *The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.