How do i define some variables in logback.xml and be able to use that variable in my java code? - logback

Question

So, I am making a custom appender and i want elasticsearch url and index pattern to be saved in logback.xml and somehow get that value in my java code and i also want to be able to override it if i want in my append() function. I am using maven for my project if that helps.
<?xml version="1.0" encoding="UTF-8"?>
<configuration debug="true">
<appender name="STDOUT"
class="ch.qos.logback.core.ConsoleAppender">
<encoder>
<pattern>%d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n</pattern>
</encoder>
</appender>
<appender name="myappender" class="esappender.EsAppender">
<m_elasticSearchUrl>http://xxx.xxx.xxx.x:xxxx</m_elasticSearchUrl>
<m_elasticSearchIndex>output-%d{yyyy-MM-dd}</m_elasticSearchIndex>
<encoder>
<pattern>%d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n</pattern>
</encoder>
</appender>
<logger name="esappender" level="debug" />
<root level="INFO">
<appender-ref ref="STDOUT" />
<appender-ref ref="myappender" />
</root>
</configuration>
I thought directly metioning it in the tags will work and i could use it in my java code but it doesn't seem to work. When i search the internet i only find variables referenced in the logback itself but i want to be able to refrence it from append() function.