This action hook is to add additional fields or data under the shipment “Remarks” in the wp-admin. This is useful for adding new additional data for Shipment.

This is a sample hook code to add Carrier Options for the Shipment, copy and paste this code to the theme functions.php file

function my_misc_custom_field( $post ){
    // Get Save post meta "_carrier"
    $carrier = get_post_meta( $post->ID, '_carrier', true );
	?>
    <p>
        <label for="_carrier">Carrie</label>
        <select id="_carrier" class="_carrier" name="_carrier" style="width:100%">
            <option value="">-- Select Carrier --</option>
            <option value="UPS Inc." <?php selected( $carrier, 'UPS Inc.' ); ?>>UPS Inc.</option>
            <option value="FedEx Corp." <?php selected( $carrier, 'FedEx Corp.' ); ?>>FedEx Corp.</option>
            <option value="XPO Logistics" <?php selected( $carrier, 'XPO Logistics' ); ?>>XPO Logistics</option>
        </select>
    </p>
    <?php
}
add_action('wpcargo_shipment_misc_actions_form', 'my_misc_custom_field');

Screenshot: