EOS-R Back Button Focus

Does anybody know how to set Back Button Focus in EOS-R?
It seems that there is no option for "focus and hold" to be assigned to AF-ON or other buttons. The moment I release the AF-ON button and try to re-compose the shot, the camera tries to obtain focus again. Is it normal to keep pushing the AF-ON during recompose? This was not the case with the 5DSR. Am I missing something?