The difference between a HEAD- and a GET-request is to tell the server to respond just with the headers or with the whole content. All other parts of the request will be the same for both methods depending on the requirements of the server. So if the server requires the hostname (e.g. due to multiple virtual hosts) you must specify HTTP/1.1 and include the host-header. Specifying HTTP/1.1, but not including the host-header is not allowed. So you could try the following.
Send-String: HEAD /testmonitor.aspx HTTP/1.0\r\nConnection: Close\r\n\r\n
Receive-String: HTTP/1.1 200 OK
If you have issues with your monitor getting green you can either sniffer the monitor traffic (in case it's unencrypted) or you make a telnet and send the above mentioned Send-String via CLI. Then you can check the response from the server and correct your settings accordingly.
Ciao Stefan :)