Issue in generating Trustpilot business generated link - BGL

Question

I am trying to generate Tustpilot Business Generated Link(BGL). I have and Encryption key and Authentication key. Ask is to encrypt json payload containing user data with AES-CBC using a key size of 256 bits, a block size of 128 bits, and PKCS7 padding mode. Below steps are mentioned in trustpilot official site as well.

1. Generate IV according to block size 128 bits
2. Encrypt the JSON with the encryptkey and IV
3. Create a signature of the ciphertext.
4. For this, we use HMAC-SHA256 and the authkey. Compute the HMAC by hashing the IV followed by the ciphertext. Here's an example: HMAC = HMAC-SHA256( IV + ciphertext )

Finally, base64-encode it like base64_payload = base64( IV + ciphertext + HMAC )

I tried this in javascript/nodejs and using crypto library to perform the encryption.

const ecryptionKey = 'xxxxx';
const authenticationKey = 'xxxxx';

const payload = {
    "email":"[email protected]",
    "name":"abc",
}
const plainText = JSON.stringify(payload);

const key = Buffer.from(ecryptionKey, 'base64');
const iv  = crypto.randomBytes(16);
const cipher = crypto.createCipheriv('aes-256-cbc', key, iv);
const HMAC = crypto.createHmac('sha256', key)
                                .update(plainText)
                                .digest('hex');

const base64_payload = Buffer.from(iv + cipher + HMAC).toString('base64')

const url = https://www.trustpilot.com/evaluate-bgl/<domain>?p= + encodeURI(base64_payload)

Only way to test this is to hit the URL and if it is asking for login to submit review, the encryption is not done correctly.

So far I have no success. Any help would be greatly appreciated as I am completely lost!