NEW since 5.1.0: Credentials for team project collections can now be configured once instead of once per job.
Automatic
Credentials will be looked up in the global configuration based on the Collection URL. Make sure you have added your credentials at Jenkins > Manage Jenkins > Configure Credentials and then add Team Project Collections that are linked to their corresponding credentials at Jenkins > Manage Jenkins > Configure System.

Consult the Global Configuration section in README.md for a walkthrough.
Manual
Credentials are specified in the job. This is the legacy behavior.