All requests to the Trellis API need to have an Authorization header like so with your unique API key: 
Authorization="YOUR_API_KEY"
To generate your API key, sign up for a Trellis account here, and navigate to the settings page (bottom left). Then click ‘copy’.

The settings page in the Trellis dashboard

The settings page on the Trellis dashboard
Your API Key is PrivateYour API key is private and shouldn’t be shared publicly. All requests to the Trellis API should be done server-side.