You'll need to head to the Mixer Xbox Page
and then stream Xbox E3 Briefing or any of the other Xbox Daily broadcasts to be eligible. Be sure to log in and link it to your Microsoft Account first!
As a way to promote Micsoft's Twitch-style streaming platform Mixer, this is a pretty good deal because you get Rare Replay, No Time to Explain, The Final Station, and DLC for Halo Wars 2 and Forza Horizon 3.
We're not sure exactly how it works or how usage is tracked but word from a number of people is that the games and content in question have started to appear in their collections the day after streaming the Xbox E3 Briefing.
We're currently streaming Day One of the Xbox Daily broadcast and will let you know if it works.
For more info head here