direct url to enable customers choose subdomain option 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:

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?