wiki uses the !VAR syntax, is the wiki wrong?
where you can this, because the wiki environment section is correct. Also, you should prefer to refer directly to the 66 documentation.
Well, use the -C options(ask to completely overwrite the environment section from the upstream file) so
# 66-enable -C -t root -S wpa_supplicant
you don't need to disable the service if it already enabled , just use the -f (force) option
# 66-enable -C -f -t root -S wpa_supplicant
Now if you want to change something at the environment variable without the need to rewrite the frontend file edit your /etc/66/conf/wpa_supplicant file or passing through the 66-env API (it's up to you). Then when you are done with your modification, just reload the service
# 66-start -r wpa_supplicant