How to pass a parameter

Where is your code that handles the querystring param? If it’s in a custom template, you may have to use that in the insert page shortcode also so that it is used to render the inserted page.

If it’s another plugin, we’ll need more information to see if there’s a workaround.

Finally, make sure that the querystring is well-formed; that encoded equals sign at the end might not need to be there? Here is what your querystring looks like parsed:

Array
(
    [?parameter] => 9as8f9283jfoeifjowiejfow=
)

For reference, here is the code that takes the shortcode attribute querystring and populates the PHP variables $_GET and $_REQUEST with the values, and also runs parse_request() from the global $wp object to get those into WordPress’s query_vars array:
https://github.com/uhm-coe/insert-pages/blob/master/insert-pages.php#L457-L468