Proxy issues with react / node.js app: not able to proxy request

I am trying to connect my react app to stripe so that users can manage their subscriptions and payments.

I am getting a little confused with the backend side of this to actually make the backend call and open the customer portal.

I'm not sure I have the backend server set up correctly. This is how I have things setup in the app.

I have a button which is intended to go to the customer portal:

<form method="POST" action="/create-customer-portal-session">
     <button type="submit">Manage billing</button>
</form>

I then have this to create the customer portal session:

const stripe = require('stripe')(Secret Key);

const express = require('express');
export const app = express();
app.use(express.static('public'));
app.use(express.urlencoded({ extended: true }));
app.use(express.json());


app.post('/create-customer-portal-session', async (req, res) => {
  // Authenticate your user.
  const session = await stripe.billingPortal.sessions.create({
    customer: 'cus_MIx5hmhlXwCOzk',
  
  });

  res.redirect(session.url);
});
app.listen(3001, () => console.log('Running on port :3001'));

Then in package.json I have the following:

"proxy": "http://localhost:3001/"

That is everything I have set up to do this currently, it returns the error:

Proxy error: Could not proxy request /create-customer-portal-session from localhost:3000 to http://localhost:3001/. See https://nodejs.org/api/errors.html#errors_common_system_errors for more information (ECONNREFUSED).

I'm getting a little lost with the proxy server side of things as it's quite new to me and I am not sure how to approach this.