To set an rDNS (reverse DNS or PTR) entry for a cloud server, you first need to log into Cloud Console. Once you're there, select your cloud server and click on "Network". You can change the rDNS entries here.
You need to be able to resolve a reverse DNS entry in both directions. This means that the domain that is assigned to an IP address must resolve to the same IP.
An IPv6 address consists of a network prefix (8 x 8 bytes) and an interface identifier (8 x 8 bytes). For a more details about how to properly write or represent an IPv6 address, please see this Wikipedia article.
First make sure which IPv6 is assigned to your NIC (network interface card, also sometimes called a network interface controller).
On this server, the interface identifier is ::1. On Cloud Console, the network prefix is already stored, so you only need to enter the interface identifier. Look at the example below. It should look similar to this:
Important warning: Make sure to use two colons where needed (::) so that you enter a valid IPv6 address.