The "label" that you see in the MultiChain Web Demo is not a built-in feature of MultiChain, but rather how the Web Demo uses the root stream in a blockchain to allow each address to identify itself by its label. Each address uses the publish command to declare its label to the networks. Other nodes obtain the label for an address by retrieving the most recent root stream item where that address is the publisher.
In general, you can use getnewaddress to create a new address, but an address cannot grant itself permissions – this has to be assigned by another address with admin or activate permissions.
