Anywhere I can find some documentation of MiniUPnP?
For example, some doc explains what the parameters of this function are.
LIBSPEC int
UPNP_AddPortMapping(const char * controlURL, const char * servicetype,
const char * extPort,
const char * inPort,
const char * inClient,
const char * desc,
const char * proto,
const char * remoteHost,
const char * leaseDuration);
theses are methods defined by UPnP standard. Have a look at http://upnp.org/specs/gw/UPnP-gw-WANIPConnection-v2-Service.pdf for UPNP_AddPortMapping.
To see more specificaly how to use with libminiupnpc, see the upnpc.c sample program. https://github.com/miniupnp/miniupnp/blob/master/miniupnpc/upnpc.c