How to describe custom property in window object with imported type in typescript 3.x

Question

How can I declare ("typescript": "3.x.x" ) that inside window object, I have some variable with some type?

E.g. I have window.foo = { bar: 'baz' }, how can I declare it? Old way of doing that was:

declare global {
  interface Window {
    foo: { bar: string }
  }
}

And it produces this error

TS2669: Augmentations for the global scope can only be directly nested in external modules or ambient module declarations.

It looks like just adding a record in some d.ts file is ok. Like this:

interface Window {
  foo: { bar: string }
}

But as I understood in such case I can't import some types in that file and use them in declaration, instead of string for example.

So what if I want to do something like that:

import { MyTypeFromSomewhere } from './somewhere'
interface Window {
  foo: { bar: MyTypeFromSomewhere }
}

How can I extended Window interface with new property and specify type for it?