WooCommerce - Add a special field to the checkout, order emails and user/order meta or Load specific custom fields on change country.

functions.php

<?php 
add_action('woocommerce_review_order_before_payment', 'gst_checkout_field');
function gst_checkout_field( $checkout ) {
    
    echo '<div id="gst_checkout_field_sec">';

    woocommerce_form_field( 'gst_no', array( 
        'type'          => 'text', 
        'class'         => array('my-field-class orm-row-wide'), 
        'label'         => __('GST No'), 
        //'placeholder'   => __('Enter a number'),
        ) );

    echo '</div>';
    
    /**
     * Optional Javascript to limit the field to a country. This one shows for italy only.
     **/
    ?>
    <script type="text/javascript">

        jQuery(document).ready(function(){
             hideShowGstField();
        });

        jQuery('select#billing_country').live('change', function(){
            hideShowGstField();
        });

        function hideShowGstField(){
            var country = jQuery('select#billing_country').val();
            
            var check_countries = new Array(<?php echo '"IN"'; ?>);
            if (country && jQuery.inArray( country, check_countries ) >= 0) {
                jQuery('#gst_checkout_field_sec').fadeIn();
            } else {
                jQuery('#gst_checkout_field_sec').fadeOut();
                jQuery('#gst_checkout_field_sec input').val('');
            }
        }
    </script>
    <?php
}
/**
 * Process the checkout
 **/
add_action('woocommerce_checkout_process', 'gst_checkout_field_process');
function gst_checkout_field_process() {
    global $woocommerce;
    
    // Check if set, if its not set add an error. This one is only requite for companies
    if ($_POST['gst_no'])
        if (!$_POST['gst_no']) 
            $woocommerce->add_error( __('Please enter your XXX.') );
}

/**
 * Update the user meta with field value
 **/
add_action('woocommerce_checkout_update_user_meta', 'gst_checkout_field_update_user_meta');
function gst_checkout_field_update_user_meta( $user_id ) {
    if ($user_id && $_POST['gst_no']) update_user_meta( $user_id, 'gst_no', esc_attr($_POST['gst_no']) );
}

/**
 * Update the order meta with field value
 **/
add_action('woocommerce_checkout_update_order_meta', 'gst_checkout_field_update_order_meta');
function gst_checkout_field_update_order_meta( $order_id ) {
    if ($_POST['gst_no']) update_post_meta( $order_id, 'gst_no', esc_attr($_POST['gst_no']));
}

/**
 * Add the field to order emails
 **/
add_filter('woocommerce_email_order_meta_keys', 'gst_checkout_field_order_meta_keys');
function gst_checkout_field_order_meta_keys( $keys ) {
    $keys[] = 'gst_no';
    return $keys;
}

/**
 * Display GST No on the order edit page
 */
add_action( 'woocommerce_admin_order_data_after_billing_address', 'gst_number_display_func', 10, 1 );
function gst_number_display_func($order){
    $order_id = $order->get_id();

    $gst_no   = get_post_meta( $order_id, 'gst_no', true );
    if($gst_no != ''){
        echo '<p><strong>GST No: </strong>'.$gst_no.'</p>';
    }
}
?>