Calculating HMAC-SHA256 Hash Value for Webhook

Calculating HMAC-SHA256 Hash Value for Webhook

I cannot figure out how to accurately calculate the HMAC-SHA256 hash for a webhook. 

Here's the documentation I'm attempting to follow.

I created a VERY simple "Raw JSON" payload in the configured webhook, but no matter what, the value I'm calculating on our side NEVER matches the value sent along with the webhook in the "x-zoho-webhook-signature" header.

See the Raw JSON payload in the webhook config of Zoho Expense. I verified that when the webhook is triggered, this same Raw JSON is included in the body. 

As I understand, all I need to do paste the Raw JSON into a calculator, such as this one: https://www.freeformatter.com/hmac-generator.html along with the secret key and I should be able to generate the same value as in the "x-zoho-webhook-signature" header. 

What am I missing here?