Access window object from in app browser


I was just wondering if it possible to access the window object when using the browser plugin?
I’m trying to build an app to extend a web service I already use, and the only way I can pull a token to verify my account is by logging in from their site. I was hoping there would be a way I could pop-up the sites login in an in-app browser, then once I log in I can read the windows local storage to get the token and bring that back into the native app.

I currently have a simple proxy server that uses puppeteer to sign in and retrieve the token, but if possible I’d like to avoid setting up a server for a relatively small project

Any help is very much appreciated!