A PTR record is the reverse DNS resolution of an IP address. In many cases it is strongly desired to have the server's IP address or addresses resolve into your domains or hostnames.
In MyHostirian navigate to Domains -> Manage DNS
Select Manage Reverse DNS on the following screen
Select Add PTR
Choose the server you want to create a PTR record for
Choose one of the IPs you want to create a PTR record for
And finally, add the reverse resolution you want to use, for example myserver.mydomain.com
You can later edit or delete the PTR record if needed