XML : How to call OpenVAS Manager service directly?

I have installed the whole OpenVAS architecture on a Debian 8 machine. I can access the OpenVAS Manager service through Greenbone Security Assistant and through the OpenVAS CLI command line, but what I need is to query the service directly by using XML, because I need to practice and to study OMP protocol itself. I know that service is running on my machine on port 9390 but when I try to call it, it never responds. What can I do? I have tried to issue to it simple queries that do not need authentication, as this: <get_version/> Or even I have tried to authenticate to it with the corresponding XML code. I have tried to call it by the Google Chrome Poster extension, by the curl command on Linux terminal, and even by a C program which uses sockets. All my attempts have failed and service never responds, although I know that it is running well, because through Greenbone Security Assistant I can play with it.