Fetch does not POST from the Client-Side to the Server-side in node.js ?

javascript is much faster than the dom, thats why you need to let the browser to render the dom first then to exec the js. it is done by this

document.addEventListener("DOMContentLoaded", function (event) {
  // content is loaded
  
  const form = document.getElementById("id-of-yow-form");
  form.addEventListener("submit", function (event) {
     event.preventDefault();
     //add yow code
  });

});

make sure to add the same id to yow form tag

 <form id="id-of-yow-form">
    <label for="name">Name: </label>
    <input id="name" name="name" type="text">

    <label for="city">City: </label>
    <input id="city" name="city" type="text">

    <button type="submit">submit</button>
  </form>