The url_response documentation mentions the following about the $host variable:
Virtual hostname: defines the virtual host and IP address of the system hosting the URL that you want to monitor. The virtual host appears in the Select variable dialog, using the $host variable. If the virtual hostname is not specified, the robot name is displayed in alarms for $host, by default.
However, I have tried using the $host variable in my alarms when the virtual hostname is not specified, and it does not display the robot name. It just contains the word "host".
url_response probe v4.56 and earlier
Defect
This will be resolved in the next version of url_response after 4.56.
If you require this functionality urgently, a test/hotfix build is attached to this KB (url_response 4.56-T1-437) which resolves the issue.