Hi!
I assume your listbox allows you to select more than one item? If that’s true, then it’s not possible using the stock listbox control.
To get what’s selected in a listbox, you use “selected option of,” which never returns more than one value even if you select multiple values, which means that Axure does not provide you with a way to figure out what’s selected in a multi-select listbox.
If you are willing to use a set of checkboxes, or if you want to make a fake listbox that looks like a real one by using multiple widgets that you toggle selection of upon clicking them, you can use this solution. (The single-facet-filter example is the one you want.)