Using AfterShip's Magento connector, AfterShip will select the carrier based on the "Title".
Simply select Custom Value and enter the AfterShip carrier slug in the "Title" field.
Please enter the exact carrier slug used in AfterShip's system.
The carrier slug is a unique code in AfterShip system for each carrier.
The slug can be found in the URL of the carrier at: https://www.aftership.com/courier
Example:
You can enter in the "Title" spain-correos-es for Spain Correos ES
https://www.aftership.com/courier/spain-correos-es