How to send data to Kinesis Data Firehose using the AWS JavaScript SDK v3?

NOTE: I cross-posted to StackOverflow here.

I am trying to send JSON data to Kinesis Data Firehose, where it will be delivered to an S3 bucket.

It works with AWS SDK v2:

const AWS = require('aws-sdk');

// Full config not shown.
AWS.config.update({
  region: 'us-east-1'
});

const Firehose = new AWS.Firehose();
const params = {
  DeliveryStreamName: 'MY-STREAM-NAME',
  Record: {
    Data: JSON.stringify({
      key: "test",
      value: "hello world"
    }),
  },
};
    
Firehose.putRecord(params, () => {});

The Data value I see in the HTTP traffic is eyJrZXkiOiJ0ZXN0IiwidmFsdWUiOiJoZWxsbyB3b3JsZCJ9 which is exactly my payload, stringified and Base64-encoded.

However, it does not work with AWS SDK v3:

import { FirehoseClient, PutRecordCommand } from "@aws-sdk/client-firehose";

// Full config not shown.
const config = {
  region: "us-east-1"
};

const client = new FirehoseClient(config);
const input = {
  DeliveryStreamName: "MY-STREAM-NAME",
  Record: {
	Data: JSON.stringify({
	  key: "test",
	  value: "hello world"
	})
  },
};

const command = new PutRecordCommand(input);
const response = await client.send(command);
console.log(response);

Now when I check the HTTP traffic, the Data value is AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA which is clearly wrong.

I have checked lots of documentation and examples (1, 2, 3), but they are not helpful.

How do I send JSON data to Firehose using AWS SDK v3?