1. 首先需要在账户A中创建ElasticSearch服务并设置相应的权限，将IAM用户的访问权限添加到该服务中；
2. 在账户B中创建一个Lambda函数，该函数会调用账户A中的ElasticSearch服务；
3. 通过AWS SDK for JavaScript等工具，使用Lambda函数中的HTTP请求，向账户A中的ElasticSearch服务发出POST请求。

示例代码:

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

const es = new AWS.ES({
  apiVersion: '2015-01-01',
  region: 'us-west-2',
  accessKeyId: 'your-access-key-id',
  secretAccessKey: 'your-secret-access-key',
  credentials: new AWS.Credentials({
    accessKeyId: 'your-access-key-id',
    secretAccessKey: 'your-secret-access-key'
  })
});

const postData = JSON.stringify({
  'key': 'value'
});

const options = {
  host: 'search--.us-west-2.es.amazonaws.com',
  path: '/myindex/mytype',
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
    'Content-Length': Buffer.byteLength(postData)
  }
};

const req = http.request(options, (res) => {
  console.log(`STATUS: ${res.statusCode}`);
  console.log(`HEADERS: ${JSON.stringify(res.headers)}`);
  res.setEncoding('utf8');
  res.on('data', (chunk) => {
    console.log(`BODY: ${chunk}`);
  });
  res.on('end', () => {
    console.log('No more data in response.');
  });
});

req.on('error', (e) => {
  console.error(`problem with request: ${e.message}`);
});

req.write(postData);
req.end();