You can auto-authenticate to Kibana via an iframe if you set up a reverse proxy. This process is described in detail in Auto-authenticating to iframe-embedded Kibana dashboard
As described in the thread, this is done by setting an Authorization header in the proxy using basic auth for a Kibana user, but you can also use an api key. ApiKey authentication via http should be enabled by default, but you can read up more on that in the docs.
API keys management falls under our free Basic license, which you can read more about here. You can get a full feature comparison showing what's included in Basic on our subscriptions page.