How to programmatically search content with a specific taxonomy term with Search API?
Yes that's right to do addCondition here's a page with some examples https://www.drupal.org/docs/8/modules/search-api/developer-documentation/executing-a-search-in-code

So you'd just need to do the machine name of your indexed taxonomy field 

$query-&gt;addCondition('my_indexed_field', $myvalue)