Note: Please copy and paste this code in functions.php of your current theme.

/*
 *For this example we are using shipment container
 *Adding dropdown field
 */

function custom_assigned_container_filter(){
  global $typenow;
  $post_type = 'wpcargo_shipment';
  if ($typenow == $post_type) {
    $all_containers = get_shipment_containers();
    if( !empty( $all_containers ) ){
      $shipment_container = isset( $_GET['shipment_container'] ) ? $_GET['shipment_container'] : 0 ;
      ?>
      <select id="wpc-user-branch" name="shipment_container">
        <option value="">Select Container</option>
        <?php
          foreach ( $all_containers as $container ) {
            ?><option value="<?php echo $container->ID; ?>" <?php selected( $shipment_container, $container->ID ); ?>><?php echo $container->post_title; ?></option><?php
          }
        ?>
      </select>
      <?php
    }
  }
}
add_action('restrict_manage_posts', 'custom_assigned_container_filter');

/*
 * Include the meta key for filtering of shipments
 */
function custom_wpcargo_shipment_query_filter( $metakey ){
  $metakey[] = 'shipment_container';
  return $metakey;
}
add_filter('wpcargo_shipment_query_filter', 'custom_wpcargo_shipment_query_filter');