Apply different boosting values when searching for phrase in Elasticsearch?

I want to search for a phrase to Elasticsearch like "personal tax date". I want the returned results to give more weight to the term "tax".

So far I know how to boost entire index or boost for different fields but still don't know how to boost different terms? Any help??

Using function score we can boost by fields

GET <index_name>/_search
{
    "query": {
        "function_score": {
           "query": {
            "query_string": {
               "query": "*personal tax date*",
               "fields": [
                "field_1",
                "field_2"
               ]
            }
            },
            "boost": "5",
               "functions": [
                          {
                  "filter": { "match": { "field": "tax" } },
                  "weight": 30
              },
              {
                  "filter": { "term": { "ent_name": "tax" } },
                  "weight": 25
              }
          ],
             "score_mode": "multiply",
            "boost_mode": "sum"
        }
    }