How to retrieve fields content when quering runtime fields from Logstasg

Elastic Stack Logstash
1

Hello
I am implementing Runtime fields.
I added my runtime fields in my Index mapping.
When searching the data from the Dev Console, i see by "fields" attribute, containing all the runtime fields I am expecting.

But, when I use the same query from Logstash to retrieve data from ES, without the _source and only using "fields", i do not see how to use the Runtime Fields values

My question is : How do I access the runtime fields in my Logstash filter section ?

Thanks