Look at the source code of the page you made that screenshot from.

I tried to copy and paste (for your information, I'm editing the official brazilian version of the site) but the box doesn't appear for some reason. Is it a plugin I have to install?

No, you'd have to go to the source code of that page, like Oot42 said above.

It would be helpful to provide a link to that wiki, as per the guidelines.

Edit: Nevermind, found it. Because it is a template, you have to copy the source code from the Template:Affiliate. Note that you'd have to upload all the images if you want it to be exactly the same.

As said above, you have to upload all the images.

The template is also missing CSS styles. They are stored here:

https://villains.fandom.com/wiki/MediaWiki:MP.css

I didn't add the images yet because they'll be different.

As for the CSS styles, I didn't know they were a thing. Is it as simple as just copy and pasting it into your own Wiki?

You can copy them to your wiki's MediaWiki:Common.css, but you have to be an admin to do this.

Alright. I've taken a break from my role as admin in that Wiki, but I'll try it on the brazilian Heroes Wiki, which I'm currently an admin at.

How exactly do I do this? The Wiki's Common.css page is empty. If I just copy and paste its equivalent from the Heroes Wiki, I'll be also be pasting the names of the staff of that Website, and we don't have a full one yet.

You copy the content of https://villains.fandom.com/wiki/MediaWiki:MP.css to your wiki's MediaWiki:Common.css.

Alternatively, you can create MediaWiki:MP.css and then import this page into MediaWiki:Common.css.

If you need further help, please link the wiki you're working on now.