Use this Elastic runtime field example to emit a static value into a new field. Using static values can be very important to make the use of dashboards more intuitive for the users of the dashboard. If a user is filtering for a specific value of a field this field and value needs to exists in any index that is used in that dashboard. Usually that happens automatically if you use the Elastic Common Schema.
If you added data sources to your dashboard that do not contain all data from the elastic common schema this could lead into problems when filtering the Kibana dashboard. Using the runtime field that emits a static value or the value from another field can help to solve that.
Here is the example for that runtime field:
The emit method emits the value that it gets in the field. This example emits just a static string value.