minimal-form.html

<script src="https://f.convertkit.com/ckjs/ck.5.js"></script>
<form action="https://app.convertkit.com/forms/FORM_ID/subscriptions" method="post" data-sv-form="FORM_ID" data-uid="UID_FROM_FORM_HTML">
  <ul data-element="errors"></ul>
  <div data-element="fields">
    <input name="fields[first_name]" type="text">
    <input name="email_address">
    <button data-element="submit">Subscribe</button>
  </div>
</form>