It is rather difficult to properly craft SIP messages in a single line (to properly pass all parameters). Therefore, the xhttp_rpc module does not support this kind of functionality. Asynchronous commands are not implemented by the xhttp_rpc module: http://kamailio.org/docs/modules/devel/modules/xhttp_rpc.html#id2521422
It is better to use the xmlrpc module for this kind of functionality. http://kamailio.org/docs/modules/stable/modules/xmlrpc.html
Regards, Ovidiu Sas