Have you tried this via the UI? The best way to master the API is to perform the desired action via the UI and eavesdrop to see which calls the UI made to accomplish the action.
I do something different, but I think is not the most optimal way (I get all the issues with the an API request but this endpoint return a lot of info and I need make a data treatment to get only the OPEN issues and get only severity).
Do you know if is any plan to get that info with an API endpoint or could be considerate?