How can I add SSH key?

You should be able to use ssh-copy-id user@host That will copy your ssh-key from the current system to host. If you have more than one key in your .ssh folder, or added via ssh-add, you can select the key you wish to copy using the -i option. e.b. ssh-copy-id ~/.ssh/id_ecdsa user@host

If you don't have ssh-copy-id, you'll have to copy the key manually

localhost$ scp ~/.ssh/id_ecdsa.pub user@remote:
localhost$ ssh user@remote
user@remote‘s password:

remotehost$ cat id_ecdsa.pub >> .ssh/authorized_keys
remotehost$ chmod 600 .ssh/authorized_keys
remotehost$ exit