Http header insert depending on host

You need to set a variable in HTTP_REQUEST to use in HTTP_RESPONSE;-

when HTTP_REQUEST {
    set fSTS 0
    if {[string tolower [HTTP::host]] eq "blah.com.au"} {
         Set flag for use in HTTP_RESPONSE
        set fSTS 1    
    }
}
when HTTP_RESPONSE {
    if {$fSTS} {
        HTTP::header insert Strict-Transport-Security "max-age=3600" 
    }
}