In order to add PTR record, please new add domain with name matching pattern below :
- IPv4 followed by ".in-addr.arpa"
- IPv6 address followed by ".ip6.arpa"
Once you add the domain you can then create PTR records with the ppropriate hostname.
Let say you have a IP address 188.8.131.52
you have to create DNS zone with REVERSED IP address in name:
184.108.40.206.in-addr.arpa (this will be a new domain in your account)
Then you have to create PTR record matching the name you want. Let say it's my_server.somewhere.com
for more detail about PTR records see https://en.wikipedia.org/wiki/Reverse_DNS_lookup