I don't get it. What is the domain key here?
I tried to read the instructions but all of it said that the signing should be done with account private key.
Ok I got confused because tried to simultaneously read how to make the certificate using Amazon AWS documentation, and seems I skipped one crucial part.
Did you figure it out? You need two different keys: the account.key and the domain.key. For most of the steps you will use the account.key to sign, but the Certificate Signing Request will use the domain.key to sign.