Get base events triggered by a threshold rule

I'm having a hard time trying to get the base events that triggered a threshold rule using a logstash pipeline. Right now I've built a KQL query that gives those documents, but still doesnt populate any field in the original event.

Code looks like:

input {
   elasticsearch { connection details to some elastic_instance_A

filter {

   mutate {
      add_field => { "field_i_need_to_fill" => "placeholder_data" }

   elasticsearch {
      tag_on_failure => '_noctlquerythreshold_exception'               
      index => "%{kibana.alert.rule.indices}"
      query => "%{kql_query_that_gives_the_data}  "
      fields => { 
        "_id" => "field_i_need_to_fill"

Edit1. Already tried using doc_field and aggregation_field options of elasticsearch filter. Still no results.