SSH login to Fortigate VM ec2 instance

I am trying to ssh log in to an aws ec2 instance with fortigate VM installed on it.

E.g.:

ssh -i ~/Documents/my_Fortigate_key.pem ec2user@ec2-xx-xx-x-xxx.eu-central-1.compute.amazonaws.com

But for some reason, I get a password request. I have tried using the instance id, which doesn't work and since I haven't set any password, I don't even know what the password might be in this case.

Logging in using admin@ec2-xx-xx-x-xxx.eu-central-1.compute.amazonaws.com works fine, but gives me different option and I am not able to access the unix system underneath fortigate. Is this by design? I am trying to ssh login directly to the instance in order to disable root authentication via ssh, so if I can't login like that by default, it would work for me.