customr.liquid

{% form 'create_customer' %}
  <input type="hidden" name="shop" value="{{ shop.permanent_domain }}">
  <div class="fields"><!-- Fields loaded from server --></div>
  <!-- Custom field with a key called "unicorn" -->
  <input type="text" name="customer[unicorn]">
  <!-- Custom field with a key called "your-custom-key" -->
  <input type="text" name="customer[your-custom-key]">
{% endform %}