Specifying index name from filebeat to logstash

Elastic Stack Beats
6

Here

 input {
  beats {
    port => 5044
  }
}

filter {
    grok {
        match => {"message" => "%{COMBINEDAPACHELOG}"}
    }
    geoip {
        source => "clientip"
    }
}

output {
  elasticsearch {

  }
  stdout { }
}

Where is the index supposed to be specified?