woocommerce-cart-input.php

<?php
/**
 * Add a text field to each cart item
 */
function prefix_after_cart_item_name( $cart_item, $cart_item_key ) {
	$notes = isset( $cart_item['notes'] ) ? $cart_item['notes'] : '';
	printf(
		'<div><textarea class="%s" id="cart_notes_%s" data-cart-id="%s">%s</textarea></div>',
		'prefix-cart-notes',
		$cart_item_key,
		$cart_item_key,
		$notes
	);
}
add_action( 'woocommerce_after_cart_item_name', 'prefix_after_cart_item_name', 10, 2 );

/**
 * Enqueue our JS file
 */
function prefix_enqueue_scripts() {
	wp_register_script( 'prefix-script', trailingslashit( plugin_dir_url( __FILE__ ) ) . 'update-cart-item-ajax.js', array( 'jquery-blockui' ), time(), true );
	wp_localize_script(
		'prefix-script',
		'prefix_vars',
		array(
			'ajaxurl'		=> admin_url( 'admin-ajax.php' )
		)
	);
	wp_enqueue_script( 'prefix-script' );
}
add_action( 'wp_enqueue_scripts', 'prefix_enqueue_scripts' );