I was able to resolve, the quickstart guide has the following set
xpack.fleet.agents.elasticsearch.hosts:
remove this from the Kibana.yaml config and recreate kibana pods
once this is removed you can edit the output in Fleet within Kibana UI (Fleet -> Settings -> Outsputs -> actions )
Add the following to "Advanced YAML Configuration"
ssl.certificate_authorities: ["/mnt/elastic-internal/elasticsearch-association/elastic-siem/elastic-siem/certs/ca.crt"]
your path may differ