Hope this would help others poking at the answer at this old post. I got the same 400 bad request status with https://reqbin.com too until I add this to the header:

aeg-event-type: Notification

Then it works both in reqbin and Postman. See the details at https://learn.microsoft.com/en-us/azure/communication-services/how-tos/event-grid/local-testing-event-grid#configure-postman