kafka mongodb sink connector not generate key value of topic

Hi, my kafka topic has key and value schemas like this :

key.avsc
{
  “namespace”: “tx.avro”,
  “type”: “record”,
  “name”: “key”,
  “fields”: [
    {
      “name”: “userid”, “type”: “string”, “default”: “null”
    }
  ]
}
value.avsc
{
  “namespace”: “tx.avro”,
  “type”: “record”,
  “name”: “value”,
  “fields”: [
    {
      “name”: “username”, “type”: “string”
    },
    {
      “name”: “result”, “type”: “string”
    }
  ]
}

via mongodb sink connector, only (username, result) are sinked.
how to sink the key (userid) of topic via mongodb sink connector ?

connector config is like this:

curl -X PUT http://connector01:8083/connectors/tx-data-connect03-1/config -H "Content-Type: application/json" -d ' {
      "connector.class":"com.mongodb.kafka.connect.MongoSinkConnector",
      "topics":"tx-data-kv-03",
      "tasks.max":"2",
      "connection.uri":"mongodb://admin:xxxxxxxx@10.10.10.10:27011",
      "database":"sytx",
      "collection":"txdata1",
      "key.ignore": "false",
      "key.converter": "io.confluent.connect.avro.AvroConverter",
      "value.converter": "io.confluent.connect.avro.AvroConverter",
      "value.converter.schema.registry.url": "http://sch1:8081",
      "key.converter.schema.registry.url": "http://sch1:8081"
} '