HostName is enabled through the automatic system macro variable: &syshostname
You can print it in the log by: %put &syshostnmae;
or get it in your code by: hostname = "&syshostname";
In some cases (as on SAS UE) you may achieve LOCALHOST as hostname.
Check documentation of system macro variables, mainly start with &sys...
some of them may differ according to sas platform.