Octokit createNodeMiddleware getting the access code from a Single page app

Question

   let url = `https://authentication-url.example.com/api/github/oauth/login`;
    this.authWindow = window.open(
      url,
      'Authorization',
      `width=500, height=500, top=300, left=300`
    );
    this.authWindow.focus();

This flow, redirects me from login, to https://github.com/login/oauth/authorize

which redirects me to /api/github/oauth/callback with the code as a query string param

The response from this is a html page which contains a token. But how can I access this token programatically. Ie, should I not use createNodeMiddleware and return a custom javascript which sends a message to window.postMessage which I can listen for in my single page application.

import {createNodeMiddleware, OAuthApp} from "@octokit/oauth-app";
const octokitApp = new OAuthApp({
  clientType: "github-app",
  // appId: APP_ID,
  // privateKey: PRIVATE_KEY,
  clientId: CLIENT_ID,
  clientSecret: CLIENT_SECRET,
});
octokitApp.on("token", async ({octokit}) => {
  const {data} = await octokit.request("GET /user");
  console.log(`Token retrieved for ${data.login}`);
});

const authApp = express();

authApp.use(createNodeMiddleware(octokitApp));
export authApp

Is how the express routes look.