To use Jabber-RPC you will need to add this module into your router in this way:

includes(Exampple.Xmpp.Rpc)

And also, add the configuration for your MyRpc module which should contains all of your public functions accesible from Jabber-RPC:

config :exampple,
  router: MyRouter,
  rpc: MyRpc

Once you have that, you can create functions inside of MyRpc and those could be called from XMPP. Keep in mind this is not controlling the security about how is using these RPC commands so, it's intended these functions should be public actions available for all of the users into the server.