In the guide there is written:
" Setting Up The Box
You will now have to log into your running box using SSH. Your cloud provider will probably give you some instructions on how to do that. If your personal computer has a command line, you’ll be doing something like this:
ssh -i yourkey.pem email@example.com
Legal note! Mail-in-a-Box is made available per the CC0 public domain dedication. By running Mail-in-a-Box, you will invoke scripts that use Let’s Encrypt to provision TLS certificates per the Let’s Encrypt Subscriber Agreement(s) & Terms of Services. Please be sure you accept the terms in both documents before proceeding.
Once inside, you will now get the Mail-in-a-Box code onto your box and start its setup. Copy and paste this into your terminal and hit enter:
curl -s https://mailinabox.email/setup.sh | sudo -E bash"
where I find “yourkey.pem”?