Not able to verify the endpoint of the webhook only App in zoom

I have created a webhook type in in zoom and trying to validate the endpoint as shown in the document below. But I am not able to get the verification done.

The URL that I am trying to validate : https://nextgen--agilebrain.sandbox.my.salesforce-sites.com/zoomRecordings/services/apexrest/sampleWebh

Hashing code :

String signature = EncodingUtil.base64Encode(Crypto.generateMac(‘HmacSHA256’, Blob.valueOf(plaintoken), Blob.valueOf(csecret)));

Reference documentation: https://marketplace.zoom.us/docs/api-reference/webhook-reference/#validate-your-webhook-endpoint

How To Reproduce
Click the validate button from the webhook only app.