Customer.io hosts services in the United States (US) and European Union (EU, host subdomains are suffixed with -eu). Select the appropriate region for server addresses that apply to your region.

1. Go to the {{< svg "connection" >}} tab and click Sources.
2. Click Add Source, pick JavaScript, and click Next.
3. Give the source a Name and copy your API Key. You'll use this key to authenticate with our API. If you don't copy the key now, you can always get it later from the Settings tab when you're done setting up your source.
4. (Optional) Test your connection by sending a test call. You can copy your write key into an app like Postman or send a CURL request. If your request is successful, and then you click Test Connection we'll let you know if your request was successful and you've set up your HTTP implementation successfully.
5. Click Submit.

Now you can connect your REST API source to destinations.

Our API uses basic authorization with a write key provided when you set up a source. If you use Postman or another platform that helps you send API calls, this write key is the Username, and the Password is blank.

Our sources are all authenticated using a Write Key that we generate when you create a source.

By default, Customer.io records a timestamp when we receive requests. If you're sending data to Customer.io in real time, you don't need to worry about the timestamp.

If you want to backfill requests, you can send a timestamp—an ISO 8601 date-time string—telling us when the request occurred. This provides a way to log track and page calls when the activities actually took place.

You only need to populate a few fields when you send data to Customer.io. Our JavaScript client and server libraries populate additional information for each call, which you'll see when you look at individual events in Customer.io.

The information below shows the complete list of information reported to Customer.io with each event.

See the endpoints below this section for the much smaller list of fields you'll actually set when you invoke methods in our client and server-side libraries.

When you send an identify call, you'll typically include a userId and traits. Most other fields are populated by Customer.io—either by a source library or our servers.

If you provide an anonymousId, we'll attribute traits to an anonymous user. If you provide both an anonymousId and userId, we'll associate anonymous activity to the userId.

Traits are the same as Attributes in Customer.io. If Customer.io Messaging is your destination, you'll see that traits map 1-to-1 with attributes. But not all systems call the things you know about people traits or attributes. You'll need to check with your destination to understand what your destination calls people-data!

When you send a track call, you'll typically include the event name and additional properties you want to track. You can provide either an anonymousId or a userId. If you provide both, we attribute the activity to the userId and ignore the anonymousId.

Most other fields are populated by Customer.io—either by a source library or our servers.