On my system, Windows 10 with ArtRage 6 on it, arscript is basically associated with ArtRage. Must have to do with this sandbox-thing of the apps from the Microsoft Store, that you can't find it. Can't you associate it by clicking on "Searching for an App on the Microsoft Store" (or so - my computer is set to german languauge)? If ArtRage Vitae doesn't show up there, type it into the Search field on the top right of the Microsoft Store Window. In my case that works. Of course I can't associate files to it (and don't want to), because I still don't owe ArtRage Vitae. But maybe it works for you.