Authenticating to iframe-embedded Kibana dashboard

Sorry for the delay, @rupaln and sorry for giving incomplete advice.

I was able to get the Kibana server to respond with a cookie header by POSTing to /api/security/v1/login with a JSON request body of

{
    "password": "<YOURPASSWORD>",
    "username": "<YOURUSERNAME>"
}

and the appropriate kbn-version: 5.1.1 header.