Find your index.php page

I can't add a website,  there is a message that says:
This is the page which is displayed when a user first visits your website. It usually lives in the public_html, the httpdocs folder or simply in a folder marked ' / ', but is unique to your web host.

then there is an input box that says: Directory path to where your website files are

I write public_html   and nothing happen,, I tried more like writing the full path or the full address but nothing happen,,, every time I click DONE bottom nothing happen,.