Hi Ilia, I'm not very familiar with scripted fields, but I found a previous thread which looks like it may be useful to you.
In this thread, the user was able to enable groovy scripts on Elasticsearch, create a scripted field, and then manually override the scripted field inside of the visualization by editing the aggregation bucket under the "Advanced" toggle: Kibana Groovy Scripted Field for Aggregation: Visualize: Unsupported script value
Hope this helps,
CJ