The thing you are asking is not popup but lightbox. For this, the trick is to display a semitransparent layer behind (called overlay) and that required div above it.
Hope you are familiar basic javascript. Use the following code. With javascript, change display:block to/from display:none to show/hide popup.
<div style="background-color: rgba(150, 150, 150, 0.5); overflow: hidden; position: fixed; left: 0px; top: 0px; bottom: 0px; right: 0px; z-index: 1000; display:block;">
<div style="background-color: rgb(255, 255, 255); width: 600px; position: static; margin: 20px auto; padding: 20px 30px 0px; top: 110px; overflow: hidden; z-index: 1001; box-shadow: 0px 3px 8px rgba(34, 25, 25, 0.4);">
<iframe src="otherpage.html" width="400px"></iframe>
</div>
</div>