The SimilarEditors extension provides an interface via MediaWiki to the Similarusers service. It is available only to users who have the similarusers right.
The service requires credentials from the extension, which are passed via the extension's SimilarEditorsClient: https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/extensions/SimilarEditors/+/bbabece604d67c7bf22bade77c538422d8385250/src/ServiceWiring.php#8
These configs need to be set to the correct values on WMF's production appservers.
This is a blocker to production deployment of SimilarEditors (but not necessarily to beta cluster deployment, since we have a mock client for testing).