You reference it as you normally would, but you won't see this working in the app or the preview because the file you're referencing is not inside BSS. There's no way around this at the moment, you just have to do it so that it works when exported.
We've been asking for a couple years + now for a way to use/reference external files within a local projects folder and have BSS recognize it, but so far no love. Hopefully we'll see this in the future not long from now.