Kibana Controls, Option List doesn't show all strings

I am trying to create an option list with wich I can select specific documents which then are dispayed in a data table. The problem is that the option list only shows strings outside of the _source field but I want to filter with fields inside the _source.
Data structure:

"_index": "index1",
"_type": "_doc",
"_id": "ikIyBHMBra6a_DzJdFae",
"_score": 1,
"_source": {
  "serialnumber": "Blibla314", //String
  "formatversion": 0,
  "username": "Production",  //String
  "commissionnumber": "",
  "datetimestart": "2018-11-08 04:00:37",
  "datetimestop": "2018-11-08 04:11:53",
  "test": "BNF1234", //String
  "resultdecimal": 0,

Only shows 3 strings:

How can I filter for username, serialnumber, test etc.? They dont show up.

I think it has something to do with "Aggregatable". But how can I set something as Aggregatable?

Found the solution. The string must be a "Keyword". Has to be set in the mappings.