IIS: Angular request causes 401 unauthorized error when calling Python Flask on another port on same server

Question

I have a legacy Angular application and there's a snippet of code below that makes a call to a ASP page on the same server and port

getTableData.getUser = function () {
    return $http.get(url)
};

However when I amend the URL to point to the same server but a different port, to a Python Flask application I get an error 401 (Unauthorized)

The Python Flask app has windows authentication. If I enable anonymous authentication, it works but obviously I don't then get the logged in user information

I followed CORS preflight request returning HTTP 401 with windows authentication but it did not work.

Any suggestions? I'm using Python 3.6 and IIS on Windows Server 2012.

The call I am making is to a Python Flask endpoint which gets the username of the person logged in.