Lighttpd mod_fastcgi respond only if conf file is using root url pattern

I have running lighttpd 1.4.59 on Lubuntu 21.10. My lighttpd.conf contains:

fastcgi.server = ( "/" => (( "bin-path" => "/var/www/html/script.fcgi", "socket" => "127.0.0.1:80", "min-procs" => 1, "max-procs" => 1, "doc-root" => "/" ))) 

There are still no problems and both 'localhost' and 'localhost/script.fcgi' are working. But if I change '"/" =>' to '"/foo" =>' then I can not get access to the /foo route because of '404 Not found' code. How to provide access to script.fcgi only if user type 'localhost/foo' in an address bar? Thanks a lot.