Trouble signing in to Humble Bundle app

I am trying to get the Humble app (Humble Games Collection) running under Bottles. It installs fine and launches too. On first launch, it asks for an account. The “sign in” link opens my default browser (outside of Bottles), which authenticates me and redirects me to a humble://login?code=... kind of URL. xdg-open does not know what to do with that though.

How can I open that URL using the Humble app that is running in Bottles?