How to add product custom fields in WooCommerce

addProductCustomFileds.php

<?php
/******************
 *  Product Hooks
 ******************/
// Print a product custom field
add_action( 'woocommerce_before_add_to_cart_button', 'printProductCustomFields' );

/***************
 *  Cart Hooks
 ***************/
// Save custom field to the art - it has 2 parameters
add_filter( 'woocommerce_add_cart_item_data', 'saveCustomFieldToCart' );

// Print the custom field in the cart
add_filter( 'woocommerce_get_item_data', 'printCustomFieldToCart', 10, 2 );

/****************
 *  Order Hooks
 ****************/
// Save custom field in the order - it has 4 parameters
add_action( 'woocommerce_checkout_create_order_line_item', 'saveCustomFieldToOrder', 10, 3 );

/**
 * Print a product custom field
 * @return void
 */
function printProductCustomFields(): void {

	// Gift Message
	printf( '<div class="container-gift-message">
			<label for="gift-message">%s</label>
			<textarea id="gift-message" name="gift_message" style="10px 20px 10px 0;"></textarea>
		</div>',
		\esc_html__( 'Gift message', 'letsgo' )
	);
}

/**
 * Save custom field to the art
 * @param array $cartItemData
 * @return array
 */
function saveCustomFieldToCart( array $cartItemData ): array {
	
	// Return early
	if ( ! isset( $_POST['gift_message'] ) ) {
		return $cartItemData;
	}

	return array_merge( $cartItemData, [ 'gift_message' => $_POST['gift_message'] ] );
}


/**
 * Print custom field in the cart
 * @param array $itemData
 * @param array $cartItem
 * @return array
 */
function printCustomFieldToCart( array $itemData, array $cartItem ): array {
	
	// Return early
	if ( empty( $cartItem['gift_message'] ) ) {
		return $itemData;
	}
	
	$itemData[] = [
		'name'  => esc_html__( 'Gift Message', 'letsgo' ),
		'value' => $cartItem['gift_message']
	];
	
	return $itemData;
}


/**
 * Print custom field to Order
 * @param WC_Order_Item $item
 * @param string $cartItemKey
 * @param array $cartValues
 * @return void
 */
function saveCustomFieldToOrder( WC_Order_Item $item, string $cartItemKey, array $cartValues ): void {
	
	// Return early
	if ( empty( $cartValues['gift_message'] ) ) {
		return;
	}
	
	$item->add_meta_data( esc_html__(' Gift Message', 'letsgo'), $cartValues['gift_message'] );
	$item->save_meta_data();
}