Add mapservice dynamically

Earlier I succesfully used the Silverlight (https://support.geocortex.com/code-snippet-group-layer-mapservice) codesample to dynamically add a service to the map.

Now I am trying to do the same thing in the HTML viewer. Adding the layer to the map is straightforward, but I am stuck at adding it to the TOC.

I think I need to create a new essentials.MapService, and then go from there. To do that in SL, there is the MapService.CreateFrom() static method, but that doesn't exist in HTML. Directly calling the constructor for a MapService needs a URL to the layers REST endpoint, which of course does not exist, since it's dynamically added...