cat ~/.ssh/config.d/srv203.conf:
Host srv203-2
User user
Hostname 203.0.113.2
IdentityFile ~/.ssh/id_ed25519_user
$ ssh-copy-id -i ~/.ssh/id_ed25519_user srv203-2
/usr/bin/ssh-copy-id: INFO: Source of key(s) to be installed: "/home/user/.ssh/id_ed25519_user.pub"
/usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
/usr/bin/ssh-copy-id: INFO: 1 key(s) remain to be installed -- if you are prompted now it is to install the new keys
user@203.0.113.2's password:
Number of key(s) added: 1
Now try logging into the machine, with: "ssh 'srv203-2'"
and check to make sure that only the key(s) you wanted were added.