Last year I had success tapping on the link in the email Apple sends out for the saved preorder. I did this on my iPhone which launched the Apple Store app on my phone. Think I had two phones purchased by 5:04am PDT (4 minutes after preorders went live).
If you don’t have the saved preorder then I would do both. Keep refreshing the Apple website and the Apple Store app to see which you can get through on first.