Dynamic Window to populate Category Description in Incident Description field?

Can anyone guide me with writing the Dynamic Window calculation syntax for this?

I have some text in my Category.Description attribute which contains details we may need from them to resolve the incident. 

When a user selects that particular Category, I'd like the DW to auto-populate the Incident.Description with text from the Category Description window, & allow them to complete the info we need.

I think this should be a fairly short calculation, but am not 100% familiar with the syntax to do so...

Thanks!