As long as Now CLI does not support being run behind a proxy (see vercel/vercel#255) you can do so with this small wrapper.
Install proxify-now:
npm i -g now global-agent proxify-now
Set your http_proxy, https_proxy and no_proxy environment variables.
Start using Now CLI with support for http_proxy, https_proxy and no_proxy:
pnow <any Now CLI commands>
proxify-now runs Now CLI with global-agent
preloaded. global-agent
adds proxy support to any http agent running in the process.