Think of Monogatari as a website first, a game engine second. Custom things like this will be achieved using web design techniques. In the case of what you're asking for, the word you'll want to google while hunting around for tutorials is "Tooltip".
https://www.w3schools.com/css/css_tooltip.asp Here's a page from w3schools.com on how to do CSS Tooltips.
Monogatari, out of the box, does not contain any way to spawn a custom element like this, so you'd need to make it yourself. Since this is a project for school, and you'd probably like to get this done relatively quickly, I recommend putting your raw HTML into Monogatari's "NVL" mode. Monogatari's dialog boxes support HTML so you can stick pretty much whatever elements you want in them, and NVL mode makes the text box occupy the entire screen so you can make your custom element appear however you like.