Button on click should ask for email before taking to a url or link

What I am trying to achieve for coupons website (https://www.dealivore.in/store/oyo-rooms-coupons) is that when someone tries to click on the button it should open a box wherein they input their email id and once submitted it should take them to the offer page.

Now this is a coupon code right now it just reveals the code to the end user.

Tried this but doesn't work

<input type="text" id="txtNumber">
<input type="button" onclick="openPage()" value="Show The Page">
<script type="text/javascript">
function openPage() {
var value1 = document.getElementById('txtNumber').value;
var url="http://www.domainname.com/" + value1 + ".pdf";
//alert(url);
window.open(url,"_blank");
}
</script>

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