[split] Codeigniter web.config index.php rewrite inside a subdirectory

from site_root/administration/application/config/config.php, any controller function shows 404 page not found. But if I place it i.e.
PHP Code:
[b]$config['index_page']='index.php';

it works fine.

I want to remove index.php from URL.

My question is,
How could I perform it through web.config ?
How could I perform it through .htaccess ?

I have asked this same question here but got no answer. I google it and found similar question here without any answer.

For your information, I am using CI 3