To enter into the infernal hordes, press "tab" on your pc. When you do, the bottom left will ask you to teleport to the endgame arena. Press it, and you'll get transported there without the mats needed to open it.
Because infernal hordes take long and isn't as rewarding, I would probably avoid it if possible.
Let me know if you wanted to know about anything else. While starting out, just continue to do the war plans.
I have to do it to progress my war plans!
I’m on Xbox tho. When I open it on map and hit the button for teleport, it acts funny.