Download the SSH Credentials Plugin. Then create an SSH Username with private key credential with Global scope.
Fill in the private key field with a private SSH Key if you wish to authenticate SSH with keys; you must leave the passphrase section blank. Your keys will need to follow the OpenSSH PEM format. See RFC 4716 for more information.

Do not password protect your SSH key.

Example private key format:
-----BEGIN RSA PRIVATE KEY-----
--------------------Some private key---------------------------- --------------------Some private key---------------------------- --------------------Some private key---------------------------- --------------------Some private key---------------------------- --------------------Some private key---------------------------- --------------------Some private key---------------------------- --------------------Some private key----------------------------
-----END RSA PRIVATE KEY-----