You cannot assign axis in HMI Operate, you must have HMI Advanced or Startup tool to carry out this operation automatically.
This is a copy-paste from Sales and delivery release:
The Software HMI-Advanced 7.6 SP2 is still required for drive commissioning.52639015
SINUMERIK Operate 2.7 SP1 and HMI-Advanced 7.6 SP2 HF2 can be installed in parallel on a SINUMERIK PCU 50.3 / PCU 50.5. However, only one of the two user interfaces can be started at one time. You can also use HMI-Advanced 7.6 SP2 or the Startup Tool 7.6 SP2 (on SinuCom DVD) for PC/PG for drive commissioning.