A question about FsBrowser Example in ARduino

Hi Guys

I have written a web server and I upload that to FsBrowser example in Arduino.

The webserver has shown successfully and It runs but for once.

I mean Every time I click on my button the browser shows me File Not Found but the URL has change as I want.

I think it is because of I dont handle that URL.

I know that I can handle that with this :

client.println("HTTP/1.1 200 OK");
client.println("Content-Type: text/html");
client.println("");

But I dont know where sholud I import this 3 lines