Create test for block page on non-existent path

Hello

I am trying to build an attack signature that will allow our team to visit a URL that does not exist on any of our websites to ensure the ASM is blocking for that site. This will also be used to test changes to the Response pages.

Is there any known way to create an attack signature that will block traversal to a path that does not exist on the server? This is the rudimentary testing signature that I put together from some documentation, but I don't seem to be getting the block\response page.

uricontent:"cmd.exe"; nocase; objonly;

Any help would be greatly appreciated.

Thanks.