Doing this right now is a bit involved. At a high level, I believe you need to implement your own CesiumSource which then generates the custom properties you want to send down to the client.
You’ll then need to tell CzmlDataSource about these custom properties on the client so that it can process them as part of the CZML document (we hope to make this part unnecessary some time soon and handle it automatically).
At that point you’ll have the data as part of your entity and you could then dynamically build the string you want on the client side from the source data.
If you were hoping to send down a description that was already preconfigured with dynamic values; interpolated strings is something that CZML doesn’t actually support yet (but it’s been on my roadmap forever).
I’ll ping someone who I know currently does this and see if they can chime in with some better instructions. If you contact AGI support they might also have a precanned example ready to go (it’s been a while since I worked on Components, so I honestly don’t know anymore).
In the future, you will have better luck asking questions like this on AGI’s official STK Components form: http://www.agi.com/agiforum/topics.aspx?ForumID=27 We try to keep the discussion of other commercial products on the Cesium forum to a minimum and the STK Components developers (including AGI support) don’t monitor the Cesium forum.
Hope that helps,