<script src="js/jquery-3.2.1.min.js"></script>
<script src="js/bootstrap.js"></script>
<script type="text/javascript">
    $(document).ready(function(){
        $('#generate').on('click', function(){
            $.get("generate.php", function(data){
                $('#coupon').val(data);
            });
            $(this).attr("disabled", "disabled");
            $('#submit').show();
        });
    });
</script>