How to get a UDT parameter's value in a Calculated Roster script of a UDT Alarm pipeline notification

When the UDT alarm is triggered, I want to get the parameter’s value on this instance of this UDT alarm in a script.

For example:
The alarm is build on this tag:
[default]Stations/Station 6/State
in the Calculated Roster of this alrm, I need to get the value of this parameter: [default]Stations/Station 6/Parameters.StationNum
How should I write the script? My script will never work:

faultedStationNumber = system.tag.readBlocking(["[.]/PARAMS/StationNum"])[0].value

There is an event argument I can use to access properties of this alarm event