Use URL parameter in post

Hi,

I would like to use URL parameter in post/pages how can I get this working?

I followed this advice:
https://www.webopius.com/content/137/using-custom-url-parameters-in-wordpress

Using step 2 code in for instance full-width.php it gave me the correct output of myvar I used in the URL (mydomain.com/page/?myvar= )

But I would like to use this in a post or page but that doesn’t seem to work. I tried 2 plugins to use PHP in posts but no luck. Any idea’s how to fix this?

Thanks.