Searching from keyword field in Logstash with query

Elastic Stack Logstash
1

I would like to filter out the string "Alert" in the message field of my index. The field is a keyword field. To achieve this, I have written the following:

filter {
    mutate {
        add_field => {"secret" => "<my_secret>"}
    }

    elasticsearch {
        hosts => ["<my_host>:9200"]
        ssl => true
        ca_file => "/etc/ssl/certs/ca-certificates.crt"
        index => "<my_index>"
        query => 'message: "Alert"'
        user => "<my_user>"
        password => "<my_password>" 
        fields => {
            "message" => "[@metadata][message]"
        }
    }

It does not achieve the desired result. I believe I am missing something fundamental.