script.liquid

{% if event.preview %}
  {% capture order_json %}
    {
      "admin_graphql_api_id": "gid://shopify/Order/12345",
      "customer": {
        "note": {{ options.tag_prefix_in_customer_note__required | append: "abc123" | json }}
      }
    }
  {% endcapture %}

  {% assign order = order_json | parse_json %}
{% endif %}

{% assign note_lines = order.customer.note | split: newline %}
{% assign desired_prefix = options.tag_prefix_in_customer_note__required %}
{% assign tag = nil %}

{% for line in note_lines %}
  {% assign line_prefix = line | slice: 0, desired_prefix.size %}
  {% if line_prefix == desired_prefix %}
    {% assign tag = line | replace: line_prefix, "" %}
  {% endif %}
{% endfor %}

{% if tag != blank %}
  {% action "shopify" %}
    mutation {
      tagsAdd(
        id: {{ order.admin_graphql_api_id | json }}
        tags: {{ tag | json }}
      ) {
        node {
          ... on Order {
            tags
          }
        }
        userErrors {
          field
          message
        }
      }
    }
  {% endaction %}
{% endif %}