it looks like you have added the powercms file in folder named admin of your site. the developer notes will strip any folder named admin to make sure the powercms/admin folder will not be compromised.
change;
include('E:/dooronsgemaakt.nl/wwwroot//WA_CMS/addtosite.php');
to:
include('E:/dooronsgemaakt.nl/wwwroot/admin/WA_CMS/addtosite.php');