Made to be used with the "Smart Coupons" WooCommerce extention. If we apply coupon from url and the coupon is dedicated to a specific product, the coupon is not applied if the product is not already in cart. So, before applying the coupon we add this product to cart.

woocommerce-add-to-cart-before-applying-coupon.php

<?php
/**
 * Add to cart before applying coupon
 *
 * Made to be used with the "Smart Coupons" WooCommerce extention.
 * If we apply coupon from url and the coupon is dedicated to a specific product, the coupon is not applied if
 * the product is not already in cart. So, before applying the coupon we add this product to cart.
 *
 * @link              
 * @version           1.0.1
 * @package           woocommerce_add_to_cart_before_applying_coupon
 *
 * @wordpress-plugin
 * Plugin Name:       Add to cart before applying coupon
 * Plugin URI:        
 * Description:       If we apply coupon from url and the coupon is dedicated to a specific product, the coupon is not applied if the product is not already in cart. So, before applying the coupon we add this product to cart.
 * Version:           1.0.1
 * Author:            Sébastien Méric <sebastien.meric@gmail.com>
 * Author URI:        http://www.sebastien-meric.com/
 * License:           GPL-2.0+
 * License URI:       http://www.gnu.org/licenses/gpl-2.0.txt
 * Text Domain:       woocommerce_add_to_cart_before_applying_coupon
 * Domain Path:       /languages
 * 
 * This program is free software; you can redistribute it and/or modify it under the terms of the GNU
 * General Public License as published by the Free Software Foundation; either version 2 of the License,
 * or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without
 * even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 *
 * You should have received a copy of the GNU General Public License along with this program; if not, write
 * to the Free Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
 **/

if ( ! defined( 'ABSPATH' ) ) {
	exit; // Exit if accessed directly
}

/**
 * Add a product to cart so applying a coupon on it from url is possible.
 *
 * @since      1.0.0
 * @package    woocommerce_add_to_cart_before_applying_coupon
 * @author     Sébastien Méric <sebastien.meric@gmail.com>
 **/
add_action( 'wp_loaded', 'woocommerce_add_product_to_cart_on_coupon_from_url' );
function woocommerce_add_product_to_cart_on_coupon_from_url() {
	global $woocommerce;

	// Made to be used with the "Smart Coupons" WooCommerce extention.
	if ( ! class_exists( 'WC_Smart_Coupons' ) ) {
		return;
	}

	// Bail early if $woocommerce is empty, or there is no coupons passed, or an empty coupons has been passed.
	if ( empty( $woocommerce ) || ! isset( $_GET['coupon-code'] ) || empty( $_GET['coupon-code'] ) ) {
		return;
	}

	// Get the coupon code.
	$coupon_code = $_GET['coupon-code'];

	// Get the coupon id or do nothing.
	if ( ! ( $coupon_id = woocommerce_get_coupon_id_from_code( $coupon_code ) ) ) {
		return;
	}

	// Get the first associated product with this coupon code or do nothing.
	if ( ! ( $product_id = intval( get_post_meta( $coupon_id, 'product_ids', true ) ) ) ) {
		return;
	}

	// Do nothing if product is already in cart.
	foreach ( $woocommerce->cart->get_cart() as $cart_item_key => $values ) {
		$_product = $values['data'];
		if ( $product_id === intval( $_product->id ) ) {
			return;
		}
	}

	// If product is not already in cart, then add it !
	$woocommerce->cart->add_to_cart( $product_id, 1 );
}

/**
 * Get a coupon ID from it's code
 *
 * @since      1.0.0
 * @package    woocommerce_add_to_cart_before_applying_coupon
 * @param      string     $code
 * @return     int
 */
function woocommerce_get_coupon_id_from_code( $code ) {
	global $wpdb;

	return intval( $wpdb->get_var( $wpdb->prepare( apply_filters( 'woocommerce_coupon_code_query', "SELECT ID FROM $wpdb->posts WHERE post_title = %s AND post_type = 'shop_coupon' AND post_status = 'publish'" ), $code ) ) );
}

?>

When i added this to my function.php I had to delete one of the ")" on line 47. Other than that it works great! Thank you for posting!

code updated :)

thanks @perrysessions

I searched long and hard and almost tried some desperate moves. But this code worked perfectly. Thank you very much, exactly what I needed!