Harmony software sometimes hides the power commands. But you should at least be able to specify that the Zone 2 device be powered on in your new Zone 2 activity. The power commands for the Zone 2 device will likely just have names like Standby, Power On, Discrete On, Power toggle, Standby toggle, etc.

There is no technical reason you can't learn from the original remote though. Have you learned other commands to your One before? It's been a long time since I've used a One, but I think the IR learning receiver is at the bottom of the remote, not the top.

In any case, you can send a request for the IR codes to onkyoadmin@us.onkyo.com , then send the desired codes to datasupport@logitech.com to be added to your account. Alternately, you can create an account at the site below and download the code list for the 626 which likely has exactly the same codes as yours.

[Link: hifi-remote.com]

I'm not clear on exactly what command(s) you're looking for, but these are my best guesses:

Zone 2 Standby
0000 006D 0022 0002 0156 00AB 0015 0015 0015 003F 0015 0015 0015 0015 0015 003F 0015 0015 0015 003F 0015 003F 0015 0015 0015 003F 0015 0015 0015 0015 0015 003F 0015 0015 0015 0015 0015 0015 0015 003F 0015 003F 0015 003F 0015 0015 0015 0015 0015 0015 0015 003F 0015 0015 0015 0015 0015 0015 0015 0015 0015 003F 0015 003F 0015 003F 0015 0015 0015 003F 0015 064B 0156 0055 0015 0E47

Zone 2 Power On
0000 006D 0022 0002 0156 00AB 0015 0015 0015 003F 0015 0015 0015 0015 0015 003F 0015 0015 0015 003F 0015 003F 0015 003F 0015 0015 0015 0015 0015 0015 0015 003F 0015 0015 0015 0015 0015 0015 0015 0015 0015 0015 0015 003F 0015 0015 0015 0015 0015 0015 0015 0015 0015 0015 0015 003F 0015 003F 0015 0015 0015 003F 0015 003F 0015 003F 0015 003F 0015 003F 0015 064B 0156 0055 0015 0E47

Zone 2 Standby/On (toggle)
0000 006D 0022 0002 0156 00AB 0015 0015 0015 003F 0015 0015 0015 0015 0015 003F 0015 0015 0015 003F 0015 003F 0015 0015 0015 003F 0015 0015 0015 0015 0015 003F 0015 0015 0015 0015 0015 0015 0015 003F 0015 003F 0015 0015 0015 003F 0015 0015 0015 0015 0015 003F 0015 003F 0015 0015 0015 0015 0015 003F 0015 0015 0015 003F 0015 003F 0015 0015 0015 0015 0015 064B 0156 0055 0015 0E47