There is (afaik) currently no way to navigate from a user's Wikimedia account to their Phabricator account. There should be either a sidebar link in Mediawiki.org userpages, or a special page redirect, or a system on Phabricator itself to lookup from a wiki username, or some combination of these.
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or a system on Phabricator itself to lookup from a wiki username
See the "MediaWiki User" field on https://phabricator.wikimedia.org/people/query/advanced/