You are an administrator of a base Jenkins master with the recommended pluginset and you...

Use a credential "SSH Username with private key", restricted to the Scope "Agents", for the node's SSH authentication.

Use a credential "SSH Username with private key", restricted to the Scope "System", for the node's SSH authentication.

You cannot use an SSH key pair. Configure a password for the agent's SSH user, and create a credential of type "Username with password" for the node's SSH authentication.

The SSH key will be stored in the )ENKINS_HOME/.ssh folder. Configure the agent to use a key from the "standard" SSH folder.