Post request to server is giving me error in express net::ERR_CONNECTION_REFUSED

I want to collect the data from a simple HTML form and send it to the server. Unfortunately, I am making some mistakes and the server is not receiving the data. The error in the console

POST http://localhost:3000/ net::ERR_CONNECTION_REFUSED

Here is my code.

server.js

var express = require("express");
var app = express();
let PORT = process.env.PORT || 3000;

// Middleware
app.use(express.static("public"));
app.use(express.json());

app.get("/", function(req, res) {
  res.sendFile(__dirname + "./public/index.html");
});

app.post("/", function(req, res) {
    console.log('kkk', req.body);
});

var server = app.listen(PORT, function() {
  console.log("Express server listening on port " + PORT);
});

I am using XMLHttpRequest() to collect the data and here is the code.

const contactForm = document.querySelector('#contactForm');
let fullName = document.getElementById('fullName');
let email = document.getElementById('emailAddress');
let phone = document.getElementById('phoneNumber');
let message = document.getElementById('message');

contactForm.addEventListener('submit', (e) => {
    e.preventDefault();
    let formData = {
        name: fullName.value,
        email: email.value,
        phone: phone.value,
        message: message.value
    }
    
    let xhttp = new XMLHttpRequest();

    xhttp.open('POST', '/');
    xhttp.setRequestHeader('content-type', 'application/json');
    xhttp.onload = function() {
        console.log(xhttp.responseText);
        if(xhttp.responseText == 'success') {
            console.log('Email sent successfully');
            fullName.value = '';
            emai.value = '';
            phone.value = '';
            message.value = '';
        }else {
            console.log('Something went wrong');
        }
    }
    xhttp.send(JSON.stringify(formData));
});

If you want to check it live.

https://contact2021.herokuapp.com/

On Server, I can see the response