How to make custom wikiboxes for countries

I'm just wondering how you are able to use country template to add in your alternate country. I want to make a fictional Reichskommissariat wiki box like this but I can't find a template. How do you do it.
Your link is broken. You have to use the source code for that wiki box from any country page with one by editing the text in a sandbox or just changing it on the article itself (click show preview and then take a screenshot, make sure to not actually save your wikibox). A sandbox requires an account.