You can register external endpoints that you want the platform to invoke on your behalf using the Registered APIs feature, found under Admin › Developer › Registered APIs. A registered API captures the base URL and authentication for a system you own, so that custom scripts and Cloud Functions can call it without embedding credentials in configuration.

Register a new API

Click Register New API at the top-right corner of the page and configure the details:

Registered API Name — a name for your registered API.
Base URL — the base URL of the API to call.
Is Read Only — enable if you want this registered API to be read-only. Note that the API will no longer be editable once it's been saved as read-only.
Headers — header names and values needed by your API (e.g. name: Authorization; value: Bearer {token}).
Authorization — the auth type and its parameters:

Auth type	Parameters
API Key	Key Name, Key Value, Add to 'Header' or 'Query Params'
Basic	Username and Password
Bearer Token	Bearer Token
None
OAuth	Grant Type, Access Token URL, Client ID, Client Secret, Scope, Send as 'Basic Auth Header' or 'Request Body'

Once you have configured your registered API and its authorization credentials, click Save. It is now available to be called from custom scripts and Cloud Functions.