setup --dashboards fails to authenticate while using an output.elasticsearch.api_key. The authentication options for the Elasticsearch username and password are automatically used when connecting to Kibana. So for consistency the api_key should also automatically be used.

For confirmed bugs, please report:

• Version: 7.10
• Operating System: all

As a workaround I found that setup.kibana.headers is an undocumented config option. Using that you can set the Authentication header manually. Note that the format is different from the output.elasticsearch.api_key because you have to use the raw header value (it does not base64 encode it for you).

setup.kibana.headers:
  Authorization: "ApiKey <base64_encoded_key>"