add authentication section

Author: pwseg

src/connections/destinations/catalog/actions-webhook-extensible/index.md

@@ -18,4 +18,55 @@ Segment maintains this destination. For any issues, [contact Segment Support](fr
 
 By default, the new destination is disabled. You'll enable it in the next section.
 
-## 2. 
+## 2. Set up authentication
+
+Before you can enable the new destination, you'll first need to choose an authentication option:
+
+1. On the new destination's page, navigate to **Settings > Authentication.**
+2. Choose one of the following authentication options:
+   - **No authentication**: Segment doesn't manage authentication.
+   - **Bearer token**: Segment automatically includes a bearer token in the API request header.
+   - **OAuth 2.0**: Segment manages the OAuth token lifecycle, including fetching and refreshing tokens.
+3. For OAuth 2.0, select one of the following flows:
+   - **Authorization code**, which requires the following fields:
+     - Client ID
+     - Client secret
+     - Authorize URL
+     - Access Token URL
+     - Refresh Token URL (usually the same as the Access Token URL)
+     - Scopes
+   - **Use client credentials**, which requires the following:
+     - Client ID
+     - Client Secret
+     - Access Token URL
+     - Refresh Token URL (usually the same as the Access Token URL)
+     - Scope
+4. Save the settings, then click **Connect** to activate the connection.
+
+You've now completed setup, and your destination is ready for event mapping and data configuration.
+
+## Mapping and data configuration
+
+1. Define the API endpoint (URL) and the HTTP method (POST, PATCH, PUT).
+2. Configure optional parameters:
+   - **Batch Size**: Specify batch size if the API supports batching entire payloads.
+   - **Headers**: Add required headers (e.g., `content-type` defaults to `application/json`).
+3. Map payload fields:
+   - Map individual fields or select a specific object from a test event.
+   - (Optional) Use a destination insert function to transform the payload according to the API specification.
+4. Send a test event to validate the setup. Debug any errors related to payload configuration or authentication.
+
+> **Note**: Segment supports batching the entire payload but not specific objects within the payload.
+
+## Known limitations
+
+- **Token Expiration Issue**: If a token expires before a test event is sent, the test event may fail with a 401 error. Workarounds include:
+  1. Using the **Event Tester** to validate connections.
+  2. Reconnecting OAuth by editing and saving settings to fetch a new token.
+
+## Enabling the destination
+
+1. Enable the destination in the **Settings** page if it is not already enabled.
+2. Save the configuration to ensure "Extensible Webhooks" is live.
+
+