<?php
add_shortcode( 'coupon_field', 'display_coupon_field' );
function display_coupon_field() {
    if( isset($_GET['coupon']) && isset($_GET['redeem-coupon']) ){
        if( $coupon = esc_attr($_GET['coupon']) ) {
            $applied = WC()->cart->apply_coupon($coupon);
        } else {
            $coupon = false;
        }

        $success = sprintf( __('Coupon "%s" successfully applied.'), $coupon );
        $error   = __("This coupon is invalid.");

        $message = isset($applied) && $applied ? $success : $error;
    }

    $output  = '<div class="redeem-coupon"><form id="coupon-redeem">
    <p><input type="text" name="coupon" id="coupon"/>
    <input type="submit" name="redeem-coupon" value="'.__('Apply coupon').'" /></p>';

    $output .= isset($coupon) ? '<p class="result">'.$message.'</p>' : '';

    return $output . '</form></div>';
}