Welcome to the Cesium Community @Kevin3811!
There are two ways this can be achieved:
- Create a custom dashed outline fabric material. It is essentially a custom shaded for your outline. For more information on fabric materials, please visit fabric documentation.
- Make the outline a Polyline, as in this example: link to example. The example was created by @kasatik143 in this discussion thread: How to create dashed circle outline? - #3 by Baruch_Levin.