Thank you @xpillons for the above information.
Please check man page of pbs_sched
-c <clientsfile>
Add clients to this scheduler's list of known clients. The clientsfile contains single-line entries of the form
$clienthost <hostname>
Each hostname is added to the list of hosts allowed to connect to this scheduler. If clientsfile cannot be opened, this scheduler aborts. Path can be absolute or relative. If rela‐
tive, it is relative to PBS_HOME/sched_priv.
**Please try this:**
1. create a file $PBS_HOME/sched_priv/myclientsfile on the PBS Server.
2. The contents of this file (myclientsfile) is as below
$clienthost scheduler.hpc.azure
$clienthost scheduler.internal.cloudapp.net
3. kill -9 pid_of_pbs_scheduler
4. $PBS_EXEC/sbin/pbs_sched -c $PBS_HOME/sched_priv/myclientsfile