---
title: "Use On API Call Starter"
slug: "transactional-journeys-use-on-api-call-starter"
description: "The On API Call starter is the entry point for transactional journeys triggered directly via an API request."
updated: 2026-01-27T10:52:06Z
published: 2026-01-27T10:52:06Z
canonical: "academy.insiderone.com/transactional-journeys-use-on-api-call-starter"
---

> ## Documentation Index
> Fetch the complete documentation index at: https://academy.insiderone.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Use On API Call Starter

> **Suggested readings**: [Transactional Journeys](/v1/docs/transactional-journeys), [On API Call Starter](/v1/docs/transactional-journeys-on-api-call-starter)

The [On API Call starter](/v1/docs/transactional-journeys-on-api-call-starter) is the entry point for transactional journeys triggered directly via an API request. It allows backend systems to explicitly enter users into a transactional journey and send messages in real time.

This starter is designed for use cases where message delivery must happen immediately after a system action, without relying on behavioral data or segmentation inside Architect.

Each API request is treated as an explicit instruction to start a journey instance.

## Step 1: Select the starter on canvas

When creating a transactional journey, the On API Call is selected as the starter on the canvas.

The starter configuration area is used to define and validate the payload structure that will be sent when the journey is triggered.

![](https://cdn.document360.io/c6df4583-da94-4cb2-bb8a-be0cbdd11109/Images/Documentation/image-20251230-121123.png)

## Step 2: Paste your payload

The payload added to the starter acts as a validation schema. It is used to verify that incoming API requests match the expected structure once the journey is active.

![](https://cdn.document360.io/c6df4583-da94-4cb2-bb8a-be0cbdd11109/Images/Documentation/image-20251229-175826.png)

The payload is validated for:

- Valid JSON format
- Presence of an idempotency key
- Target journey definition
- Target user structure
- Dynamic attributes

If any required part of the payload is **missing** or **invalid**, the API request is **rejected,** and users do **not** **enter** the journey.

### Dynamic attributes and dynamic content

Dynamic attributes defined in the payload are exposed as Dynamic Content within channel elements.

These attributes are used to personalize transactional messages, such as including order details, user information, or action-specific data.

> [!NOTE]
> Dynamic attributes must be **explicitly** **whitelisted** on the starter. Only whitelisted attributes can be used inside Email or SMS content. If a dynamic attribute is not whitelisted on the starter, it **cannot** **be** **referenced** in channel elements, even if it is sent in the API request.

![](https://cdn.document360.io/c6df4583-da94-4cb2-bb8a-be0cbdd11109/Images/Documentation/image-20251229-154537.png)

### Preview dynamic content

The **Preview** section lets you see how dynamic content will be rendered within channel elements.

The Preview option helps confirm **attribute names**, **placeholder** resolution, and **message** **structure**. It does **not** **trigger** the journey or **send** messages.

![](https://cdn.document360.io/c6df4583-da94-4cb2-bb8a-be0cbdd11109/Images/Documentation/image-20251229-154642.png)

## Step 3: Copy journey ID

Each transactional journey has a **unique** Journey ID. You must copy this Journey ID and include it in your API requests to target the correct journey.

If a journey is **changed**, **duplicated**, or **relaunched**, a **new** **Journey ID** may be generated. In such cases, API requests must be updated with the new Journey ID to use it on your actual API request.

## Step 4: Test API

The **Test API** option allows you to test the payload structure before sending real requests. It does not add users to the journey or send messages to end users.

The Test API:

- Validates the request payload.
- Returns success or failure responses.
- Indicates missing or invalid fields.
- Surfaces authorization and validation errors.

![](https://cdn.document360.io/c6df4583-da94-4cb2-bb8a-be0cbdd11109/Images/Documentation/image-20251229-154929.png)