How can I set up a wildcard (*) subdomain?
Unfortunately, it is not possible to bind wildcard hosts with IIS on a shared hosting environment. You need to have a dedicated machine with a dedicated IP that handles all incoming traffic, without being bound to a specific domain (i.e. default IIS website handling all incoming traffic).
AFAIK, this feature is not supported in IIS even in version 8.5.
On the DNS side, make sure it's set as a CNAME * pointing to @.
If your website is hosted at WebsitePanel you probably need to use the "Add Web Site Pointer" to make the A record know which Website to use.
(You might also need to remove the previously created A record.)