Setting Chef Server URL for chef-metal

How do I set the chef server url passed to a new machine created by

I’ve tried this:
with_chef_server Chef::Config[:chef_server_url],
client_name: Chef::Config[:node_name],

which doesn’t do anything to client.rb (ie leaves it at I’ve
also tried this:

with_chef_server node[‘provisioner’][‘chef_server_url’], {
:client_name => machine_name,
:signing_key_filename => Chef::Config[:client_key]

which just throws an error. Is chef-metal properly documented somewhere? I
have no idea where most of these variables are coming from.