Generate keys and disable password login

On client 

ssh-keygen -t rsa -b 4096 -f ~/.ssh/share_key

On server

adduser $USER

Copy key to server

ssh-copy-id -f -i .ssh/sharekey.pub user@target.com

Create the Proxmox user

In the Proxmox web UI:

Go to Datacenter → Permissions → Users → Add.

Add adminuser@pam (auth realm = PAM).

Then assign permissions:

Go to Datacenter → Permissions → Add → User Permission.

User = adminuser@pam

Path = /

Role = Administrator

Test if logins work!

/etc/ssh/sshd_config

PasswordAuthentication no
PermitRootLogin no