In order to use RDNS (Reverse DNS), do the following:
1. Go to the control panel of your service.
2. Choose Network tab.
3. In the IP sections (IPv4 and/or IPv6), you will find the Reverse DNS button.
Note: the domain name must have a corresponding forward record (A or AAAA) already set at the domains DNS provider that points to the IP (v4 or v6).