We have Opensearch indexes with a flat_object field mapping. I’d like to be able to search for documents with certain words within this mapping

e.g. if the document is

PUT myindex/_doc/5
{
  "header": {
    "messageId": "5"
  },
  "payload": {
    "error": {
      "field1": "field1value",
      "field2": "words i'd like to be able to search on"
    }
  }
}

and the “payload” field is a flat_object

the following query finds the document

get myindex/_search
{
  "query": {
    "match": {
      "header.messageId": 5
    }
  }
}

the following query finds the document

GET myindex/_search
{
  "query": {
    "match": {
      "payload": "field1value"
    }
  }
}

the following query finds the document

GET myindex/_search
{
  "query": {
    "match": {
      "payload": "words i'd like to be able to search on"
    }
  }
}

the following query finds nothing

GET myindex/_search
{
  "query": {
    "match": {
      "payload": "search"
    }
  }
}

I can only find the document using field2 if i exactly match the string, what’s the best way to enable searching for words within the string ?