Handle com.unity3d.kharma for web-based Asset Store integration?

[cassidyjames]

This may be a duplicate of #67 and #92, but those were closed without any further details.

I want to test using Unity for a project, so I found this free sample project on Unity's web-based Asset Store. After adding it to my account, there's a button to "Open in Unity". When I hit that, I'm presented with an "Open With..." dialog because the com.unity3d.kharma scheme is unhandled:

Unfortunately when trying to manually set this up in a .desktop file, the Hub opened but the asset did not, so it's possible this would need to be fixed within the hub code itself.