How can we integrate Prometehus alertmanager to pagerduty

[Rupal]

Hello all,

I have recently started using Prometheus and I am leveraging Prometheus alertmagaer to trigger alerts on Pagerduty.

I created service on Pagerduty of type Events Api v2 and configured the integration key on alertmanager yaml file. But the error I see is 

Failed to post message to Pagerduty v1: POST \"https://events.pagerduty.com/generic/2010-04-15/create_event.json\" :Forbidden

Can someone please assist if any config is required to be set to enable authentication?

Thanks,

Rupal