> ## Documentation Index
> Fetch the complete documentation index at: https://docs.go.gbgplc.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Step 2: Start a journey
> Create a new interaction journey instance using the v2 API.
Once you have an access token, you can start a journey instance. This creates a new execution of the published journey and returns an `instanceId` that you use in all subsequent API calls.
## Before you start a journey
Before sending the request, make sure you have:
* **Published hosted journey**: A [published hosted journey](/docs/go-v2/platform/journey-builder/publish-journey) in preview or production environment. You need the journey's resource ID and version to start a journey instance.
* **Access token**: obtained in [Step 1: Authenticate](/docs/go-v2/low-code-builder/execution-guides/authenticate) for API authentication.
* **Journey Resource ID**: Specific journey Resource ID. Click **Dashboard** in the GBG GO platform to find the Resource ID for your published journey.
* **Journey version**: Specific journey version number. Click **Dashboard** in the GBG GO platform to find the version number for your published journey.
## Choose how to start the journey
There are two execution modes:
| Mode | Description |
| ------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| **Partial prefill mode** | You supply domain elements you already have upfront in the `context.subject` object when starting the journey. The remaining domain elements are collected from the end user through the hosted journey page. |
| **Non-prefill mode** | You start the journey with an empty `subject` object. All required data is collected from the end user through the hosted journey page. |
Use partial prefill mode when you already have some verified user data. Use non-prefill mode if the end user will provide all their data during the hosted journey.
### Start a journey (non-prefill mode)
If you are not including any user data, send an empty `subject` object along with the delivery configuration:
```bash BASH theme={null}
curl --request POST \
--url https://eu.platform.go.gbgplc.com/v2/captain/journey/start \
--header 'Authorization: Bearer your_access_token' \
--header 'Content-Type: application/json' \
--data '{
"resourceId": "97c8b0c5272ba450c48899cc78f6e3f3ba89b5d131c42f7ea972588bd8213b3e@y5z6bj7o",
"context": {
"config": { "delivery": "page" },
"subject": {}
}
}'
```
#### What happens
* A journey instance is created.
* The response includes an `instanceId` and `instanceUrl` that you use in subsequent steps.
* All required domain elements are collected from the end user through the hosted journey page.
### Start a journey (partial prefill mode)
If you already have customer data that you don't want to ask the end user for again through the hosted journey, you can supply it upfront when starting the journey. To do this, remove those domain elements from the [**Interaction Group** panel](/docs/go-v2/low-code-builder/interactions/explore-interactions) in the journey builder. This moves the domain elements to the start interaction, which means they can be supplied through partial prefill in the `context.subject` object when you start the journey.
For example, if your journey collects the customer's full name and date of birth, and you already have it, remove `FullName` and `DateOfBirth` from the interaction group.
It then moves to the start interaction.
To see the start interaction in the journey builder, click the **Start of journey** node in the Journey builder. This shows the domain elements that can be prefilled when starting the journey. In this example, `FullName` and `DateOfBirth` can be prefilled.
You can include it in the `context.subject` object instead. For example:
```bash BASH theme={null}
curl --request POST \
--url https://eu.platform.go.gbgplc.com/v2/captain/journey/start \
--header 'Authorization: Bearer your_access_token' \
--header 'Content-Type: application/json' \
--data '{
"resourceId": "97c8b0c5272ba450c48899cc78f6e3f3ba89b5d131c42f7ea972588bd8213b3e@y5z6bj7o",
"context": {
"config": { "delivery": "page" },
"subject": {
"identity": {
"firstName": "Jane",
"lastNames": ["Doe"],
"dateOfBirth": "1992-10-01"
}
}
}
}'
```
The structure of the `context.subject` object must follow the domain element schema paths configured for your journey. Always check the schema before sending data. Providing data in the wrong structure results in validation errors or incomplete journeys. To check the required fields and structure for your journey, navigate to **Dashboard** in the GBG GO platform, click the **Actions** list for your journey, and click **View schema**. This shows the expected data structure for the `subject` object.
#### What happens
* A journey instance is created with the prefilled data applied.
* Domain elements that match the prefilled data may already be satisfied, so the end user won't be asked for them again on the hosted journey page.
* Any remaining required domain elements are collected from the end user through the hosted journey page.
## Request body fields
| Field | Required | Description |
| ------------------------- | -------- | ------------------------------------------------------------------------------------------------ |
| `resourceId` | Yes | The unique identifier for the published journey, in the format `{hash}@{version}`. |
| `context.config.delivery` | Yes | The delivery mode. Set to `"page"` for hosted journeys. |
| `context.subject` | Yes | An empty object `{}` for non-prefill mode, or structured identity data for partial prefill mode. |
## Successful response
If your request is successful, the API returns a `201 Created` response:
```json JSON theme={null}
{
"instanceId": "P7U7Licy40cLZiENe8MrsL",
"instanceUrl": "/journey/P7U7Licy40cLZiENe8MrsL",
"status": "started",
"message": "Journey P7U7Licy40cLZiENe8MrsL started successfully"
}
```
### Response fields
| Field | Description |
| ------------- | ------------------------------------------------------------------------------ |
| `instanceId` | Unique identifier for this journey instance. Use this in all subsequent calls. |
| `instanceUrl` | The URL path for the hosted journey page. |
| `status` | Indicates the result of the start request. |
| `message` | Human-readable confirmation message. |
Save the `instanceId`, you need it for every subsequent API call in this journey.
Hosted journey sessions are single-use and cannot be resumed. If the end user closes or leaves the hosted journey page, the session cannot be continued. The session URL is also valid for a limited time. If the session expires or is abandoned, then start a new journey instance to generate a fresh URL.
## Best practices
* Always confirm that the journey has been published before attempting to start it.
* Use the exact `resourceId` for your target environment. Resource IDs differ between staging and production.
* For partial prefill mode, always retrieve and validate against the schema before sending data. Incorrect data structures result in validation errors.