Follow below steps to add a custom attribute to the Shipping Method in Magento 2,

Step 1. Access your Magento 2 Admin Panel.

Step 2. Click on Stores in the left sidebar.

Step 3. Expand the Sales section and click on Shipping Methods.

Step 4. Find and select the shipping method to which you want to add a custom attribute.

Step 5. Look for the configuration fields for the shipping method.

Step 6. After configuring your settings, click Save Config to apply changes.

Note: Adding custom attributes often requires custom code. You may need to create or modify a custom module to add new attributes and update the shipping methods.