Documentation

allows the base URL for confirmation links to be changed. This can be useful when blogs are in site subdomains, for example your site URL is example.com but your blog is at example.com/blog

function change_confirm_link( $link ) {
    $link = 'http://www.example.com/blog/?s2=';
    return $link;
}
add_filter('s2_confirm_link', 'change_confirm_link');