What is the right way to make a batch request to bing ads content api using nodejs?

Question

Hi, I am trying to make a batch request to insert a product using bing ads content api: [https://learn.microsoft.com/en-us/advertising/shopping-content/products-resource using nodejs. However, I am getting an 400 error code.
I think the issue is how I am defining the Item object this is the code I am using :

  var refreshToken = await CustomerManagementService.instance.refreshToken;
  var customerId = await CustomerManagementService.instance.getUserId();
  var accountId = await CustomerManagementService.instance.GetAccountId();

  var authToken = await CustomerManagementService.instance.getAuthToken();

  const product = {
    availability: "in stock",
    brand: brand,
    channel: "Local",
    condition: "brand new",
    contentLanguage: "en",
    imageLink: imageLink,
    link: link,
    offerId: offer_id,
    price: {
      currency: "USD",
      value: 0.00,
    },
    targetCountry: "GB",
    title: title,
  };

  var productJson = JSON.stringify(product);


  var item = {
    batchId: 1,
    product: productJson,
    method: "insert",
    merchantId: process.env.STORE_ID,
  };
  
  JSON.stringify(item);
  var items = [];
  items.push(item);

  JSON.stringify(items);
  var url = 'https://content.api.bingads.microsoft.com/shopping/v9.1/bmc/' + process.env.STORE_ID + '/products/batch';
  console.log(`url is ${url}\n`);


  const options = {
    "method": "POST",
    "url": url,
    "headers": {
      "Content-Type": "application/json",
      "AuthenticationToken": `${authToken}`,
      "CustomerAccountId": `${accountId}`,
      "CustomerId": `${customerId}`,
      "DeveloperToken": process.env.DEVELOPER_TOKEN,
    },

    data: items,
  }

  await axios(options)
  .then(response => {
    console.log(response.data);
  }).catch(error =>{
    console.log(error.message);
  });

Can you give me an example of how the Item object should be defined or whether there is another problem with my code. Thank you in advance! Have a wonderful day!