How can I update _score based on the attribute value in Search Suggester Query

Question

My requirement is to update _score based on an attribute. Below is my request:

{
  "_source": {
    "excludes": [],
    "includes": [
      "somefield1",
      "fqn_completion"
      "somefield2"
    ]
  },
  "suggest": {
    "suggest_fqn": {
      "completion": {
        "field": "fqn_completion.US",
        "size": 10,
        "skip_duplicates": true
      },
      "prefix": "unit"
    },
    "suggest_fqn_alt1": {
      "completion": {
        "field": "fqn_alt1_completion.US",
        "size": 10,
        "skip_duplicates": true
      },
      "prefix": "Kansas"
    },    
    "suggest_fqn_alt2": {
      "completion": {
        "field": "fqn_alt2_completion.US",
        "size": 10,
        "skip_duplicates": true
      },
      "prefix": "Kansas"
    }
  },
  "track_total_hits": true
}

And below is response of the above request:

{
    "took": 3,
    "timed_out": false,
    "_shards": {
        "total": 1,
        "successful": 1,
        "skipped": 0,
        "failed": 0
    },
    "hits": {
        "total": 0,
        "max_score": null,
        "hits": []
    },
    "suggest": {
        "suggest_fqn": [
            {
                "text": "unit",
                "offset": 0,
                "length": 4,
                "options": [
                    {
                        "text": "UNITY,WI,United States",
                        "_index": "index name",
                        "_type": "_doc",
                        "_id": "asdasdasdasads",
                        "_score": 1.0,
                        "_source": {
                            "somefield1": "something",
                            "somefield2": 12345,
                            "fqn_completion": {
                                "US": "UNITY,WI,United States"
                            },
                            "fqn_alt1_completion": {
                                "US": null
                            },
                            "fqn_alt2_completion": {
                                "US": null
                            }
                        }
                    }
                ]
            }
        ],
        "suggest_fqn_alt1": [
            {
                "text": "Kansas",
                "offset": 0,
                "length": 6,
                "options": [
                    {
                        "text": "UNITY TWP,PA,United States",
                        "_index": "index name",
                        "_type": "_doc",
                        "_id": "asdasdasdasads",
                        "_score": 1.0,
                        "_source": {
                            "somefield1": "something",
                            "somefield2": 12312,
                            "fqn_completion": {
                                "US": "UNITY TWP,PA,United States"
                            },
                            "fqn_alt1_completion": {
                                "US": null
                            },
                            "fqn_alt2_completion": {
                                "US": null
                            }
                        }
                    }
                ]
            }
        ],
        "suggest_fqn_alt2": [
            {
                "text": "Kansas",
                "offset": 0,
                "length": 6,
                "options": []
            }
        ]
    }
}

I want to update _score with somefield2 value. I thought of use function_score but how to use it in above request that I couldn't figure it out. Anyone has any idea how can I achieve it?