Hi. I have figured it out. It is presented as "mysensor:MyMQTT/4/255/V_:state:default". No number in current version from mysensors!!!
The next step was to edit MyMQTT.cpp: char V_50[] PROGMEM = ""; into char V_50[] PROGMEM = "50";. Now I get battery status via "mysensor:MyMQTT/4/255/V_50:state:default".

So in conclusion I have answered my own questions and have solved them...