direct url to enable customers choose subdomain option

http://docs.whmcs.com/Linking_to_WHMCS shows different ways to link to a product in whmcs.

"To link to the order process with a subdomain preselected, you can use a link like this:

http://demo.whmcs.com/cart.php?a=add&pid=1&domainoption=subdomain&sld=google
"

however, it does not seem to work when using pro sites and whmcs mu provisioning. I would like a direct link that can link to my trial product using the subdomain option instead of domain registration/transfer.

what url format can be used?