Redirecting hosted sites on availablily (not servers)

Actually, i've ran aground a bit with creating the monitor for the pool. I can write an iRule to choose a pool based on the domain name, but what syntax can I use to specify a host header to send in the monitor? So that I can determine if a particular site (member of a pool) is up or down?

 

 

An ordinary http monitor doesn't seem to let you specify a host header, so I assume I'd have to use a scripted monitor, but even then the examples I've start with a GET request, i've not seen a way to request a page from dog.com rather than cat.com (both served on port 80 on the same physical server)