~ az vm extension set -h
[...]
Examples
Add a new linux user
az vm extension set -n VMAccessForLinux --publisher Microsoft.OSTCExtensions --version 1.4
--vm-name myvm --resource-group yugangw --private-config '{"username":"user1",
"ssh_key":"ssh_rsa ..."}'
``
I'm a bit against this stuff. The docs can be a little bit more generic/formal.