asp.net MVC 5 cancelation token

I have an MVC 5 project that when a user runs it, a pop-up alert box show up; with the option to cancel the process. If the user clicks "cancel" the cancelation token gets invoked. The issue is, if the user close the browser it also invokes the cancelation Token. I want a work around for this issue using Javascript or C#. Expected behavior: only cancel the process if user clicks cancel. If user close the browser, do not invoke the cancelation token; this will keep the process running while the browser is closed.