On beta cluster, I have added the cawiki and eswiki new wiki. I followed the instruction at https://wikitech.wikimedia.org/wiki/Nova_Resource:Deployment-prep/Add_a_wiki#Step_3_:_Setup_Search to populate the search index. Since they are not created yet, the script spurt a couple Warning that you might want to catch to report a nice error message.
deployment-bastion:~$ mwscript extensions/CirrusSearch/maintenance/forceSearchIndex.php --wiki cawiki --batch-size=50
PHP Warning: Search backend error during sending 1 documents to the content index after 42. IndexMissingException[[cawiki_content] missing] [Called from CirrusSearch\ElasticsearchIntermediary::failure in /mnt/srv/scap-stage-dir/php-master/extensions/CirrusSearch/includes/ElasticsearchIntermediary.php at line 97] in /mnt/srv/scap-stage-dir/php-master/includes/debug/Debug.php on line 303
Warning: Search backend error during sending 1 documents to the content index after 42. IndexMissingException[[cawiki_content] missing] [Called from CirrusSearch\ElasticsearchIntermediary::failure in /mnt/srv/scap-stage-dir/php-master/extensions/CirrusSearch/includes/ElasticsearchIntermediary.php at line 97] in /mnt/srv/scap-stage-dir/php-master/includes/debug/Debug.php on line 303
[ cawiki] Indexed 1 pages ending at 1 at 2/second
Indexed a total of 1 pages at 2/second
Version: unspecified
Severity: normal