What you’re looking for is called position: fixed and it’s a CSS property. You can read more about CSS positioning here
position: fixed makes the element stay in the same position (relative to the viewport) even if you scroll down.
I’m not really sure what you’re trying to accomplish here but if you only want an alert box to open when you click on a button, then you’ll need an event listener. You can read more about those here
Is there any specific problem that you’re trying to solve that we can help you with?