You can do something like this
METHOD-1
<input id="myInput" placeholder="Enter some text" name="name"/>
const input = document.querySelector('#myInput');
input.addEventListener('input', updateValue);
function updateValue(e) {
// here you can call your pop-up window
}
METHOD-2
<input type="text" id="myInput" oninput="updateValue()">
function updateValue() {
// here you can call your pop-up window
}