Maybe that is why, I am using a header {“Authorization”: "Basic <Base64 encoded UserName:API Key>"} for my header for all my POSTs and GETs and this works fine in the HTTP Plugin with the Campaign route.
According to their Documentation on Campaign API - Proofpoint, Inc.
Security
Each request:
- MUST use SSL.
- MUST use service credentials to authenticate to the API.
- MUST use the HTTP Basic Authorization method.
- MUST use the HTTP GET method