Subdomain feild on order form

I would like to add a custom field on my order form for a subdomain. I can easily add the field under "Custom Fields" for the product.

However is there a way I can make it so if someone has already chosen the name it will inform them the name has already been chosen? I'm going to manually register the subdomain myself and I would just need the field to check and see if anyone else currently has that name in WHMCS.

I have already tried the "Subdomain Options" here: http://docs.whmcs.com/Products_and_Services#Other and this does not do what I am looking for.