How to link non prefixed files not in the home directory?

How do I link a non-dot file not in the home directory.

Note, I have a few regular dotfiles that are working correctly.

I want to link `custom.js` in a folder called `~/.jupyter/custom/` and Im trying it like this:
```
externals = {
    'custom.js': '~/.jupyter/custom'}
```

But Im getting issues. What is the proper way to do this?

Thanks,
Derek