How to create a simple custom shortcode in Wordpress?

[admin]

I created a folder in the

/wp-content/plugins

directory called customshortcode. The <a href="https://codex.wordpress.org/Function_Reference/add_shortcode" rel="nofollow noreferrer">add_shortcode command</a> should link the shortcode with the function.

In

/wp-content/plugins/customshorcode/

I created 2 files:

main.php

and

test.php

Contents of main php is:

<?php require_once('../../../wp-includes/shortcodes.php'); ?>

<?php function custom_shortcode_func() {
  include_once('test.php');
}

add_shortcode('customshortcode','custom_shortcode_func');

?>

And test.php simply displays a division block:

<style>.testdiv{border:1px solid black; width:300px; heigh:300px;}</style>

<div class="testdiv"></div>

I first run the

/wp-content/plugins/customshorcode/main.php

file to see no errors and simple white screen. I assumed the short-code should be saved.

But when I enter [customshortcode] in the page, I don't get the division displayed, instead the text [customshortcode] instead.

I think I somehow need to link the page type to the plugin or something like that. Could you help me out?