Set your HTML form's action attribute to your form's action URL.

Your form's action URL is

<form action="">
  <input type="email" name="email" />
  <button type="submit">Subscribe</button>

Ensure all input, select and textarea elements inside your form have a name attribute, otherwise you will not receive the data filled in these fields.

Make sure your form contains a button element of type "submit".

Make sure you replace the placeholder form id with your own form id.

You can create own form id here.