Custom simple field and error in using script_fields

Elastic Stack Elasticsearch
1

Hi I would like to append to my search results just custom simple field with value, what is being changed in app process this field I need to append on that specific search results set. What I found was script fields, but is really needed to use "script_field" to just append simple static value equal for each result of one result set ?

Also if I would like to use script_fields to add field the PHP library is causing error

array(3) {
  ["body"]=>
  array(3) {
    ["query"]=>
    array(1) {
      ["bool"]=>
      array(1) {
        ["should"]=>
        array(1) {
          [0]=>
          array(1) {
            ["term"]=>
            array(1) {
              ["user_id"]=>
              string(5) "26431"
            }
          }
        }
      }
    }
    ["from"]=>
    int(0)
    ["size"]=>
    int(100)
  }
  ["script_fields"]=>
  array(1) {
    ["degree"]=>
    array(1) {
      ["script"]=>
      string(1) "1"
    }
  }
  ["_source"]=>
  array(1) {
    [0]=>
    string(1) "*"
  }
}

Elasticsearch\Common\Exceptions\UnexpectedValueException: script_fields is not a valid parameter

Thanks for advice.