Url Rewrite always redirects to .html

I would like to create an URL-Redirect for a Page I created via controller in a custom module.
I tried creating an Url-Rewrite for this purpose, let’s say my Url-key is “urlkey” and I’m trying to apply the redirect to a category-page.
However after saving the Rewrite it automatically adds a .html ending to the urlkey.
How can I make the Rewrite redirect to just “urlkey”?