I have added a custom field in WooCommerce checkout like below

function oc_add_checkout_fields( $fields ) {
 $fields['shipping_sub_area'] = array(
    'label'        => __( 'Sub Area' ),
    'type'        => 'select',
    'class'        => array( 'form-row-wide', 'update_totals_on_change' ),
    'priority'     => 68,
    'required'     => true,
    'id'            => 'th_sub_area',
    'options'           => array(''=>'Select',
                            'Clifton' => 'Clifton1', 
                            'DHA' => 'DHA1',),
    'custom_attributes' => array(),
    'validate'          => array(),
    'default'           => '',
);

return $fields;
}
add_filter( 'woocommerce_default_address_fields', 'oc_add_checkout_fields' );

When this field is changed, a pre loader is shown on the your order section in checkout form. I want to perform following: If selected value in above field is Clifton1 then add shipping charges 100 to the cart else add shipping charges 300 to the cart,

I have tried different hooks and actions but i am unable to get the value of custom added field inside the hook. For example I have used following filter:

function filter_woocommerce_cart_shipping_packages( $array ) { 
    // make filter magic happen here... 
    //die('I am hre');
   echo '<pre>';
   print_r($array);
  return $array; 
}; 
     

// add the filter add_filter( 'woocommerce_cart_shipping_packages', 'filter_woocommerce_cart_shipping_packages', 10, 1 );

In $array, i am getting:

[destination] => Array
            (
                [country] => PK
                [state] => TA
                [postcode] => 
                [city] => fds
                [address] => fsdf
                [address_1] => fsdf
                [address_2] => fsd
            )

But i am not getting value of 'shipping_sub_area' or th_sub_area.