Extend window object and write d.ts files for tsc

we have a javascript project and want to move to typescript.
In the current setup, we have a global object (let’s call it myGlobal) with variables and functions that we are accessing either through windows.myGlobal.myFunction or directly as myGlobal.varA.

As expected, tsc would complain that windows.myGlobal does not exist on the window object.

My guess was to write a global.d.ts declaration file for tsc like this:

interface myGlobal {
  myFunction: any
}


// Extend the Window interface
interface Window {
  myGlobal: MyGlobal;
}

// Declare myGlobal as a global variable
declare let myGlobal: MyGlobal;

and even though I have

  "include": [
    "**/*.ts",
    "**/*.js"
  ],

in my tsconfig, it still complaints about the same error.
Can someone help?