I'm afraid I have no idea what you are doing. You already had the requests coming to your Kamailio this morning when you tried using dispatch_rpc() inside your xhttp event route. You posted an error log line.
I don't know what the "http postman" is and what it does. All I know is that my example code is coming from a Kamailio in my setup where it is working.
I guess you need to use tcpdump or ngrep to see what's going on.
Sebastian