How do I programmatically apply a coupon in Woocommerce for first order made by a customer?

In Woocommerce I'm trying to find a way to apply a 20% discount to the first order made by a new customer. It appears that I can use the functions woocommerce_after_checkout_validation and check_new_customer_coupon to do this but it doesn't work.

Here is my code from function.php:

add_action('woocommerce_after_checkout_validation','check_new_customer_coupon', 0);

function check_new_customer_coupon(){
    global $woocommerce;
    // you might change the name of your coupon
    $new_cust_coupon_code = 'test';

    $has_apply_coupon = false;

    foreach ( WC()->cart->get_coupons() as $code => $coupon ) {
        if($code == $new_cust_coupon_code) {
            $has_apply_coupon = true;
        }
    }

    if($has_apply_coupon) {

        if(is_user_logged_in()) {
            $user_id = get_current_user_id();

            // retrieve all orders
            $customer_orders = get_posts( array(
                    'meta_key'    => '_customer_user',
                    'meta_value'  => $user_id,
                    'post_type'   => 'shop_order',
                    'numberposts'=> -1
            ) );

            if(count($customer_orders) > 0) {
                $has_ordered = false;

                $statuses = array('wc-failed', 'wc-cancelled', 'wc-refunded');

                // loop thru orders, if the order is not falled into failed, cancelled or refund then it consider valid
                foreach($customer_orders as $tmp_order) {

                    $order = wc_get_order($tmp_order->ID);
                    if(!in_array($order->get_status(), $statuses)) {
                        $has_ordered = true;
                    }
                }

                // if this customer already ordered, we remove the coupon
                if($has_ordered == true) {
                    WC()->cart->remove_coupon( $new_cust_coupon_code );
                    wc_add_notice( sprintf( "Coupon code: %s is only applicable for new customer." , $new_cust_coupon_code), 'error' );
                    return false;
                }
            } else {
                // customer has no order, so valid to use this coupon
                return true;
            }

        } else {
            // new user is valid
            return true;
        }
    }

}

via Chebli Mohamed