The configuration of the @PLUGIN@ plugin is done in the gerrit.config file.
[plugin "@PLUGIN@"]
url = http://example.org/avatars/${user}.jpg
changeUrl = http://example.org/account.html
sizeParameter = s=${size}x${size}
lowerCase = true
Please note that http:// URLs will be automatically rewritten to https://, if gerrit.canonicalWebUrl uses HTTPS.