弹出表单以输入新提交 - 限制IE 8

I'm trying to make this pop up view for inputting data, basically a PHP submission form. However, it seems that I am hit with many restrictions during my internship. I am only allowed to create something that works on IE 8. They want everything on one page, and I am stuck with the pop-up submission form.

Can anyone advise me on how to go around doing it? This is my current script.

    <script>
        function myFunction() {
            var x;
            var site = prompt("Please enter the details of the submission");
            if (site != null) {
                x = "Your idea: " + site + " has been successfully submitted!";
                document.getElementById("demo").innerHTML = x;
            }
        }
    </script>

and this is my calling function

    <button onclick="myFunction()">Prompt Box</button>
    <p id="demo"></p>

Please advise!