add_filter('woocommerce_checkout_fields','new_input_field_classic_checkout');

function new_input_field_classic_checkout($fields) {
    $fields['billing']['additional_info'] = array(
        'type' => 'text',
        'label' => 'Additional info',
        'priority' => 121,
        'required' => false,
        'class' => array('form_row_wide'),
        'placeholder' => '',
    );
    return $fields;
}