Select a Juniper router/switch (Routing Engine/control-plane host resources) as the target host to create a monitor — the SNMP service should be up and running on it. Click New Monitor, then check SNMP Custom on the Favorites tab, click Next, and confirm the host. On the next page, click Select... to open the built-in SNMP MIB Browser and type jnxHrStoragePercentUsed into the Find box to locate it in the OID tree, selecting the specific row/instance you want to monitor since this is a table column, then select it and click OK. The amount of the storage represented by this entry that is allocated, as a percentage of the total amount available. On the monitor's Main parameters page you can set the target's SNMP port (default 161), credentials, polling interval, and other settings — see the SNMP Monitor help for details. On the State conditions and Alerting tabs, configure when the monitor should change state and trigger an alert; since this is a Gauge32-type OID, Value bounds is the most useful condition here — trigger an alert if the value drifts significantly from its normal baseline, since an unexpected change often points to a real underlying issue. Click Finish to create the monitor; you can adjust any parameter later.