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How to access custom attributes of a particular work item?

This is the API URI I'm using and able to get data with this condition:
https://myjazzserver:9443/jazz/rpt/repository/workitem?fields=workitem/workItem[projectArea/name=Defect Management]/(id|summary|(creator/name))
However, there is a custom attribute named "External ID" that I want to extract to, and when I embed it in the url just like how i embedded "creator name", I get not a valid field name error. Please help in resolving the error. Thank you!
One answer

Referring to the discussion here:
If your custom attribute is string type, it would look like:
https://myjazzserver:9443/jazz/rpt/repository/workitem?fields=workitem/workItem[projectArea/name='Defect Management']/(id|summary|(creator/name)|(stringExtensions[key='External ID']/value))
More custom type may refer to: https://jazz.net/wiki/bin/view/Main/ReportsRESTAPI#workitem
I hope this helps.