Aria Operations for Logs 8.x
Only standard static fields are available in the forwarding filter because it is run before post processing in Aria Operations for Logs for custom content. This is why the filters are limited to regex expression. More info can be found here: Add a VMware Aria Operations for Logs Log Forwarding Destination
Extracted fields that are custom added, are not available for use in the Log Filter and Log Forwarding Filter.
The field lists in Aria Operations for Logs filters are created dynamically based on logs that have been ingested.
Do a search in the Explore Logs Section