If you open up the writer feature type parameters, look for this setting:
Table > General > Type Mapping Creation Parameters > Geometry Fields
You should find a table of geometry fields and their types. Simply change the type of your geometry field from `geo_point` to `geo_shape`; this will cause the Elasticsearch writer to store your polygons as GeoJSON polygons rather than as the center point of the shape.
Certain Elasticsearch functionality can only be used with either `geo_point` or `geo_shape` geometry fields, so be sure to have a look at the Elasticsearch documentation on this subject (geo_point docs, geo_shape docs)