Credentials have been created and can be found in /data/project/wmde-uca-test/.elasticsearch.ini.

When the write request is granted - do I need to change anything else than $wgCirrusSearchClusters to make it work on the test wiki?

I actually don't know what config will be needed. The information on how to access the Elasticsearch cluster is at https://wikitech.wikimedia.org/wiki/Help:Tool_Labs/Elasticsearch. The servers in this cluster run on a non-standard port and require HTTP Basic Auth for write access. CirrusSearch may or may not support these constraints. I would not be surprised to learn that you need to make some local patches to CirrusSearch to get it running. I would suggest poking around in the configuration a bit and then reaching out the nice folks in the #wikimedia-discovery irc channel if you need more help.