WooCommerce – How to add a custom field to only a specific email template [closed]

Best way would be to save a custom version of the theme template into your theme folder. That way you can customise that individual email and include the custom field.

Can do this by copying the customer-completed-order.php from WooCommerce into a /woocommerce/templates/ folder in your theme.

More info on the WooCommerce docs: Template Structure + Overriding Templates via a Theme