通过Facebook共享包含URL参数的URL

I've got the following form set up to allow customers to share a URL with a URL parameter attached to it:

<form action="http://www.facebook.com/sharer.php" method="GET" target="_blank">
    <input type="text" id="u" name="u" value="https://www.example.com/page/">
    <input type="text" id="code" name="code" placeholder="Enter your code here">
    <button type="submit" id="submit">Submit</button>
</form>

However, Facebook strips the ?coupon_code=XXXXXXX URL parameter. Any ideas how I would get around that?

Solution below- combine fields using JS:

<script type="text/javascript">
    function funraiser_url() {
        var url = document.getElementById('url').value;
        var coupon = document.getElementById('coupon').value;
        document.getElementById('u').value = url+"?coupon="+coupon;
    }
</script>

<form action="http://www.facebook.com/sharer.php" method="GET" target="_blank" enctype="application/x-www-form-urlencoded">
    <input type="text" id="url" name="url" value="https://www.example.com/shop/">
    <input type="text" id="coupon" name="coupon" placeholder="Enter your full code here">
    <input type="text" id="u" name="u">
    <button type="submit" id="submit" onclick="funraiser_url();">Submit</button>
</form>