Short snippet that shows how to get url parameters in drupal . to get query parameter form the url, you can us the following.
If you have the url for example /path?id=x
To get "id" from the url, use:
$id= \Drupal::request()->query->get('id');
This is how you can dynamically modify the content displayed in the "empty" area of your "custom" view when it's rendered as a block, based on the configuration's empty header text. Of course you can update the conditions as you think they fit better for you. You can put this into your web/modules/custom/custom.module file.
Although this is of course available from within views admin ui section, sometimes in production for example you might not provide for security reasons the views_ui module making it hard for modifying the empty header text message of a view.
