Link from subdomain-folder of YOURLS to main domain with CORS issue

[areopagites]

I would like to create a page like this with YOURLS functionality:
http://yourwish.es

But I have a subfolder like "https://example.com/urls/"
Is it possible to get access from the main domain "example.com" to the subfolder "/urls/" with YOURLS?
I have copied the url bar and the "add_link()"-button to the html-website on the main domain.
All script sources link to the subfolder like this:
<script src="https://example.com/urls/js/common.js?v=1.7.9" type="text/javascript"></script>

On my website all browsers ignored the click on the "add_link()"-button at the main domain, but when I try it on "https://example.com/urls/admin" everything works.
So I try to solve the CORS-issue with a line in config.php:
<?php header("Access-Control-Allow-Origin: http://example.com Vary: Origin");

And in functions-html.php:
function yourls_content_type_header( $type ) { yourls_do_action( 'content_type_header', $type ); if( !headers_sent() ) { $charset = yourls_apply_filter( 'content_type_header_charset', 'utf-8' ); header( "Content-Type: $type; charset=$charset" ); header("Access-Control-Allow-Origin: YOURLS_SITE"); return true; } return false; }
But nothing happens.
What can I do?

[dgw]

Is this actually a CORS issue?

Needs more information like version numbers, httpd, platform, browser, and most importantly logs from server & browser console.

[areopagites]

Only the jquery.cal.js with the undefined variable "l10n_cal_month" is listed as an issue in the browser console, now.
But I think it would also work without this variable.
Before I fixed the CORS issue, this was the result of the console:

You can see the result of my main website: https://fancythatshit.com
But the button doesnt work.
I try it with the latest versions of Chrome, IE and Opera.
Webhosting-Server: Debian, Version "Buster"
Webserver: nginx and Apache