How can I enable Ubuntu 16.04 ESM?

# Note that the following steps are not necessary in Ubuntu Pro
# Install the latest UA client
$ sudo apt update
$ sudo apt install ubuntu-advantage-tools

# Use the client to attach this machine to your contract using your UA token
$ sudo ua attach <token>
# Ensure ESM-infra is enabled as well:

$ sudo ua enable esm-infra
$ sudo apt update
$ sudo apt upgrade