It seems that in the current iteration of the Extensions API we can read highlighted marks but not set them.
I'm not well-versed in the difference between highlighted and selected marks, but our viz builders are saying there is a difference, and that sounds right given that there are two separate getter methods already for Worksheets. So ideally we would like to see the same setter methods for highlighted marks that selected marks already have:
Great idea Plamen!
You should post it in the ideas forum so we can keep track of it!