Setup URL redirection/rewrite for numbers

If I have a domain name like "example.com" hosted on IIS. How can I setup so any URL request with a numeric argument such as example.com/ would be redirected to for instance example.com/function/trigger/? (where "function/trigger" are hardcoded paths)

What is the way to do this? In my index.html, I could add a Javascript function that during load looks for a numeric parameter in the URL and then via javascript calls a redirect method. However, then the user needs to see/flicker the index.html before getting redirected. I would like that this is instant and not recognized that a redirected occurred.