foo.json

PUT /_component_template/mappings
{
  "template": {
    "mappings": {
      "properties": {
        "@timestamp": {
          "type": "date"
        }
      }
    }
  }
}

PUT /_component_template/empty
{
  "template": {}
}

PUT /_index_template/together
{
  "index_patterns": ["foo"],
  "template": {
    "mappings": {
      "properties": {
        "@other-timestamp": {
          "type": "date"
        }
      }
    },
    "settings": {
      "index.number_of_shards": 3,
      "index.number_of_replicas": 0
    }
  },
  "composed_of": ["mappings", "empty"]
}

PUT /foo

GET /foo

################# output

{
  "foo" : {
    "aliases" : { },
    "mappings" : {
      "properties" : {
        "@other-timestamp" : {
          "type" : "date"
        },
        "@timestamp" : {
          "type" : "date"
        }
      }
    },
    "settings" : {
      "index" : {
        "routing" : {
          "allocation" : {
            "include" : {
              "_tier_preference" : "data_content"
            }
          }
        },
        "number_of_shards" : "3",
        "provided_name" : "foo",
        "creation_date" : "1628781621962",
        "number_of_replicas" : "0",
        "uuid" : "ApcnEgLjSWeTvAQyBeZVZA",
        "version" : {
          "created" : "7130499"
        }
      }
    }
  }
}