$pssh->add_host($secondary_server,
                gateway => { proxies => "ssh://$primary_server" },
                ... );

##</code><code>##

$pssh->add_host($secondary_server,
                gateway => { proxies => { url => "ssh://$primary_server",
                                          user => $primary_user,
                                          password => $primary_password, ... } },
                ... );