This code adds the subid parameter if does not exist to the current URL.
Use this plugin to insert the php code:
https://wordpress.org/plugins/my-custom-functions/
But you need to use add_action() to call this function somewhere.
function my_change_url() {
global $wp;
$current_url = home_url(add_query_arg(array($_GET), $wp->request));
if ( empty($_GET['subid']) ) {
$cuser = wp_get_current_user();
$username = $cuser->user_login;
$current_url = add_query_arg(array($_GET), $wp->request);
$current_url = add_query_arg('subid', $username);
$current_url = home_url($current_url);
}
return($current_url);
}
OR
You may use a shortcode:
add_shortcode('affiliate_subid', 'my_change_url');
Put the short code anywhere on a page:
[affiliate_subid]