Nginx proxy server receiving dynamic proxy_pass in one location but unsure how to provide it in subsequent locations

This is my first outing with nginx. I am using it as a workaround for a quirk of AWS ElasticSearch Kibana dashbaords, namely that I need to use an iframe and Cognito authentication does not work in iframes.

My problem is that, while I have a hard coded one off solution, trying to create a generalized solution is proving difficult.

I need my nginx server to be able to serve as proxy for one of several (properly configured) elasticsearch kibana dashboards the user can choose from.

I currently have the following kibana.conf file. When I pass the elasticsearch kibana dashboard url with my server ip address (ex. http://XX.XXX.XXX.XX/{elasticsearch kibana dash url}.us-east-1.es.amazonaws.com/_plugin/kibana/), the first location can pull the dash url from the request uri and set the proxy_pass to the correct address.

However, on the second location, which is required because the Kibana dashboard process requires a flurry of further calls, those are all requests such as http://XX.XXX.XXX.XX/_plugin/kibana/app/kibana. The way it is currently, with the kibana dashboard url hardcoded in proxy_pass, it works beautifully. But I have no idea how to pass the dashboard url dynamically the way I did with the first call.

Any insight would be greatly appreciated. Thank you!

server {

  listen 80;
  server_name XX.XXX.XXX.XX;

  # error logging
  error_log /var/log/nginx/kibana_error.log info;

  location ~ ^/(?<kibana_dash_url>.+)/_plugin/kibana/$ {

    proxy_http_version 1.1;
    proxy_set_header X-Real-IP XX.XXX.XXX.XX;
    proxy_set_header Connection "Keep-Alive";
    proxy_set_header Proxy-Connection "Keep-Alive";
    proxy_set_header Authorization "";

    proxy_pass https://$kibana_dash_url/_plugin/kibana/;
    proxy_redirect https://$kibana_dash_url/_plugin/kibana/ http://XX.XXX.XXX.XX/kibana/;

    }

      location ~ (/app/kibana|/app/timelion|/bundles|/built_assets|/translations|/es_admin|/plugins|/api|/ui|/elasticsearch) {
         proxy_pass              https://<hardcoded elasticsearch url here>.us-east-1.es.amazonaws.com;
        # proxy_pass              https://<no idea how to get dynamic elasticsearch url here>.us-east-1.es.amazonaws.com;
        proxy_set_header        X-Real-IP $remote_addr;
        proxy_set_header        X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header        X-Forwarded-Proto $scheme;
        proxy_set_header        X-Forwarded-Host $http_host;
        proxy_set_header        Authorization  "";
    }
}