inbound webhook not working for me

Hi, very curious about this plugin, however seem to struggle to get it to work. It feels to me that I have followed the instructions and I’m getting a success message, however that message is a bit ambigous:

[unable to copy/paste the response here in its entirety, unfortunately – breaking it down]

“success”:false,”data”:
“send_to”:””,”subject”:””,”message”:””,”headers”:[],”attachments”:
“msg”:”The subject argument cannot be empty.”

this is the URL used (domain and key redacted): https://domain.com/?wpwhpro_action=inbound_deal&wpwhpro_api_key=keyj&action=send_email&[email protected]&wpwhpro_subject=newDeal&message=budget:500000,tenure:sales,phone:+23423423234234,email:[email protected]

Could you please help me understand what I’m missing out on?