“wpc_after_shipment_details_metabox” is an action that allows you to add custom fields in the receiver details section.

Copy and paste the following code in your current theme’s function.php

function mycustom_shipment_details_meta( $post_ID ){ ?>
    <tr>
    <th><label>
        <?php esc_html_e('Custom Field','wpcargo'); ?>
      </label></th>
    <td><input type="text" id="wpcargo_custom_field" name="wpcargo_custom_field" value="<?php echo esc_html( get_post_meta($post_ID, 'wpcargo_custom_field', true) ); ?>"size="25" /></td>
  </tr>
  <tr>
<?php
}

add_action( 'wpc_after_shipment_details_metabox', 'mycustom_shipment_details_meta', 99 );

Screenshot: