I wanna build a plugin that only works when I have an extra character at the end of the url input in the browser

Question

I am buiding a plugin that modifies the html inputs for pages (layer)  when the user adds a specific extra character at the end of the page url on the browser

example: https://www.question2answer.org/qa/20321/best-way-to-override-qa_db_points_calculations-function/myspecificextrachar

 how can I add the control over the url input