Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
It will be there when you open the app, just click to complete order, review the information, confirm and you're done.
Yup, that ^^^ As long as you don’t need to make last second changes, just open app, click confirm, done.

What worked for me: Starting 5 minutes prior to open, launch app. It will say “soon” or something like that. Close app (swipe out), reopen app. Keep doing until it’s up and you see your order. Then confirm.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.