How to dynamically populate the description field

Hi,
Does anyone know how I can dynamically populate the description field
of a work item, be a defect, task or story, with some text. The
content that's to be rendered on the Description field will varies
based on the value set in Filed Against field.
So for instance, if the Filed Against is set to, say, Base OS, the
following text are displayed:
Image Name:
Version #:
OS Type:

So, if this can be done, are there any documentations for implementing
this feature?

There is an early preview version of custom default values available in
2.0.0.2. The 3.0 version will be improved in this area.

https://jazz.net/wiki/bin/view/Main/AttributeValueProviders

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Regards,
Patrick
Jazz Work Item Team