The procedure to create a macOS Sierra recovery usb key is the same as it was for El Capitan:
To create your macOS Sierra recovery key you need to download the installer from the App Store. After the download has finished quit the installer and plug in your USB key (at least 8 gb). Now you can create your recovery key with the following command.
be aware to use the correct volume name
sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/*VOLNAME* --applicationpath /Applications/Install\ macOS\ Sierra.app --nointeraction