setup the page for whmcs and use encoded url

Hi
I have set the plugin up to use a page and i can see the page and it shows some whmcs stuff.

Now i want to link to a order page with products on it but that redirects to the main page.
url to encode:
https://clients.beltex.nl/cart.php?gid=8
Gives
http://www.snelgoedkoop.nl/wordpress-webdesign/self-care/selfdervice/cart.php/?gid=8

But theni need to log in, what i offcourse not want for new customers, any help on that