SF_14.4 - https://www.progress.com/documentation/sitefinity-cms/configure-output-cache

web_config_cache.xml

<configuration>
  <appSettings>
    ...
    <add key="sf-env:outputCacheConfig:optionalHeaders" value="Key" />
    <add key="sf-env:systemConfig/outputCacheSettings/cacheService:authenticationKey" value="SECRET_VALUE" />
    ...
  </appSettings>
</configuration>