How to specify the elasticsearch index with logstash forwarder

Question

What I want to achieve:

• configure from logback-spring.xml or application.properties the elasticsearch index in which the logs will end up.

What I tried:

• I am currently using this dependency:

<dependency>
            <groupId>net.logstash.logback</groupId>
            <artifactId>logstash-logback-encoder</artifactId>
            <version>7.3</version>
        </dependency>

With this logback.xml:

<?xml version="1.0" encoding="UTF-8"?>
<configuration debug="true">

    <appender name="LOGSTASH" class="net.logstash.logback.appender.LogstashTcpSocketAppender">
        <encoder class="net.logstash.logback.encoder.LogstashEncoder"/>
        <destination>the-logstash.com:5000</destination>
    </appender>

    <root level="INFO">
        <appender-ref ref="LOGSTASH"/>
    </root>

</configuration>

And on the logstash side, I am using this configuration:

output {
  stdout {
    codec => rubydebug
  }
 
  elasticsearch {
    hosts => ["localhost:9200"]
    index => theindex
  }
}

Observation, I can see the logs in elasticsearch under the index theindex

Issue:

• Now, if I want to change the destination index, I need to modify directly the file and restart logstash.

Question:

• How can configure the destination index of the logs from the application.properties with something like (this is an example) spring.logstash.index=someotherindex or directly from the logback.xml?