At the part you see a spartan with the camera spinning, press start and IMMEDIEATLY spam the B Button. The halo wars 2 title will momentarily freeze, then skip to the lobby for you to play. If if dosent work, rinse and repeat.
HIGH SUCCESS RATE
Your welcome 
Please spread the word.
This is how they release a beta
Hope they patch it mid - way of the beta.
> 2533274962122285;2:
> This is how they release a beta
It’s an Alpha they coined as a Beta. I don’t know about you but I ran into a severe bug where it cause all apps on the Xbox One to stop working until restarted.