[Plugin: Query Multiple Taxonomies] Append string to returned URL

I have been trying to manipulate the return URL for dropdown results, but nothing is working. I even tried hardcoding a URL for qmt_url function in my functions.php but that doesn’t seem to change how the results are returned (maybe it only works for lists!?).

The code I think should work, but doesn’t:

function my_qmt_url() {
    return '/';
}
add_filter( 'qmt_url', 'my_qmt_url' );

Anyway, can someone confrm if returned dropdown URLs can be manipulated. I basically just want to append a variable to the end of the returned URLs, and the closest I ever got would always add it to the start of the base URL and thus try to digest it as part of the query (which is not what I want) … where as I just want the Taxonomy Query plugin to pass it’s current content, but at the stage of requesting the URL, I just want to append something to the URL.

I know this will work because if I hardcode my var at the end of current returned URLs in the address bar, the submitted dropdown result page works as I want.

https://www.ads-software.com/extend/plugins/query-multiple-taxonomies/