Set required input checkout field

functions.php

/**
  Update functions.php into child theme
  this script works for
  http://www.iograficathemes.com/documentation/woocommerce-iva-field-premium/
  See: https://docs.woothemes.com/document/tutorial-customising-checkout-fields-using-actions-and-filters/
**/

remove_action( 'woocommerce_billing_fields', 'wciva_custom_billing_fields'); 
add_action( 'woocommerce_billing_fields', 'override_cf');
function override_cf( $fields ) {
    $fields['billing_cf'] = array(
            'label'         => esc_html__('Codice Fiscale/P.IVA', 'wc-iva'),
            'placeholder'   => esc_html_x('Codice Fiscale/P.IVA', 'placeholder', 'wc-iva'),
            'required'      => true,
            'class'         => array('form-row-first'),
            'clear'         => true
            );
    return $fields;
}