script.liquid

{% if event.preview %}
  {% assign order = hash %}
  {% assign order["admin_graphql_api_id"] = "gid://shopify/Order/1234567890" %}
  {% assign order["note_attributes"] = array %}
  {% assign order["note_attributes"][0] = hash %}
  {% assign order["note_attributes"][0]["name"] = "Existing attribute name" %}
  {% assign order["note_attributes"][0]["value"] = "Existing attribute value" %}
{% endif %}

{% assign attributes = array %}

{% for pair in options.default_order_attributes_to_add__keyval_required %}
  {% assign attribute = hash %}
  {% assign attribute["key"] = pair[0] %}
  {% assign attribute["value"] = "" | append: pair[1] %}
  {% assign attributes[attributes.size] = attribute %}
{% endfor %}

{% for note_attribute in order.note_attributes %}
  {% assign attribute = hash %}
  {% assign attribute["key"] = note_attribute.name %}
  {% assign attribute["value"] = note_attribute.value %}
  {% assign attributes[attributes.size] = attribute %}
{% endfor %}

{% action "shopify" %}
  mutation {
    orderUpdate(
      input: {
        id: {{ order.admin_graphql_api_id | json }}
        customAttributes: {{ attributes | graphql_arguments }}
      }
    ) {
      order {
        customAttributes {
          key
          value
        }
      }
      userErrors {
        field
        message
      }
    }
  }
{% endaction %}