Zoho CRM function invoked by REST API Key does not receive a POSTed payload from Zoho Sign's webhook feature

Zoho CRM function invoked by REST API Key does not receive a POSTed payload from Zoho Sign's webhook feature

Posting a payload to the URL of the API Key invokes my function, but I can't access the POSTed payload data inside my function.

I have confirmed that a payload is being POSTed correctly by testing with my own hosted webhook.

I have tried receiving the payload as a String or Map and the function receives nothing.

What am I doing wrong here? Or is it not possible to receive a POST payload in this way?

If it's not possible I will have to use my own hosted webhook, but I'd prefer not to have to.

Here is my function:

string standalone.RentalAgreementSigned(Map objJsonPayload)
{
mapPayload = objJsonPayload.toMap();
strNotifications = mapPayload.get("notifications");
sendmail
[
from :zoho.adminuserid
to :"EMAILHERE"
subject :"RentalAgreementSigned() triggered"
message :strNotifications
]
return strNotifications;
}

Thanks.

Marcus