Using the HTTP plugin
basic auth creds MUST be principal:secret from where you made new API creds in the TAP dash.
Secret creds MUST be the secret key from principal:secret (seems silly no?)
baseURL MUST be : https://tap-api-v2.proofpoint.com
type MUST be basic auth
SSL = YES
finally your GET call MUST be like this (an example)
/v2/campaign/ids?interval=2021-03-09T00:00:00Z/2021-03-10T00:00:00Z&page=1&size=100