We have published a new extension that allows you to add a “hint” for your custom field at Account and Address locations.
More details: https://www.opencart.com/index.php?route=marketplace/extension/info&extension_id=32138
The name of a custom field is often not enough to inform a customer what the entry is for or the required format. This extension gives you an extra field to better describe the purpose and validation requirements for all of your custom fields. The field hint is displayed in all relevant guest and customer forms. See the images included on this page for examples.