How to make router's log-in pop-up?

Question

When I go to my router's log-in page(192.168.xxx.xxx), there is a non-HTML and non-CSS pop-up. How do I make it?

Answer 1

I would suspect a pop up is actually simply an alert, created in JavaScript

Since you ask how to make it, An alert is created like:

alert("Hello world");

However, I would guess yours will be more "prompt" like, something like

<script>
function myFunction() {
    var ip = prompt("Please enter your IP", "");
}
</script> 

JSFIDDLE