index.js

const handleFormSubmit = event => {
    event.preventDefault();

    const form = document.getElementById('contactForm'); // 1
    const formData = new FormData(form).entries(); // 2

    var xmlhttp = new XMLHttpRequest(); // 3 
    var url = "https://<URL_website>/<caminho>";  // 4
    xmlhttp.open("POST", url); // 5
    xmlhttp.setRequestHeader("Content-Type", "application/json;charset=UTF-8"); // 6
    xmlhttp.send(JSON.stringify(Object.fromEntries(formData))); // 7

};