add_filter( 'woocommerce_email_order_meta_fields', 'custom_woocommerce_email_order_meta_fields', 10, 3 );

function custom_woocommerce_email_order_meta_fields( $fields, $sent_to_admin, $order ) 
{
    $fields['ic'] = array(
        'label' => __( 'NO KAD PENGENALAN PESERTA' ),
        'value' => get_post_meta( $order->id, 'ic', true ),
    );
    return $fields;
}