A custom page template in subdirectory

Hello.

I would like to create a custom page template in subdirectory. For examplein …wp-content/themes/mytheme/page-templates/my-template.php

I discovered the following attached code, but I do not know how to use it.

In which file I have to paste the code below? functions.php?

Thanx for help.

add_action('template_redirect', 'template_overrides');
function template_overrides()
{
    if( is_page('potd') ):
       include(HOMEPATH . '/potd_/potd.php');
       exit;

    //fallthrough to default
    endif;
}