Sending timely and personalized messages to individuals or small groups is critical to delivering a strong customer experience and maintaining engagement. Transactional messages—such as One-Time Passcodes (OTPs), billing updates, or activity confirmations—allow you to share meaningful, real-time updates from your server. This guide explains how to send transactional messages (push, email, or SMS) with OneSignal’s API using custom data and user identifiers.

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Common use cases

Use transactional messages to:
  • Send login and verification codes (OTP)
  • Confirm orders, receipts, or subscription changes
  • Deliver billing status or renewal alerts
  • Remind users about appointments or deadlines
  • Acknowledge key actions (e.g. signups or purchases)

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Requirements

Before sending transactional messages, we suggest reviewing the following guides:

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Identifying users

To target individual users, you must identify them within OneSignal. The recommended approach is to set an External ID, which should map to the user identifier used in your database or CRM. OneSignal also supports up to 20 aliases per user, enabling you to associate multiple identifiers (e.g., other_user_id, facebook_id, etc.) across your systems. For email and SMS, you can also send messages directly using the email address or phone number respectively.

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Targeting users

Use the Create Message API to send transactional messages across push, email, and SMS channels by targeting users via aliases, email addresses, phone numbers, or subscription IDs. Use include_aliases to target the recommended external_id or other aliases like so: 
{
  "app_id": "YOUR_APP_ID",
  "include_aliases": {"external_id": ["userA", "userB"]},
  "contents": {"en": "English Message"},
  "target_channel": "push"  
}

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Send to subscriptions

If you want to send to specific Subscriptions, you can use the include_subscription_ids property. This option is not recommended because Users can have multiple Subscriptions. 
{
  "app_id": "YOUR_APP_ID",
  "include_subscription_ids": ["1dd608f2-c6a1-11e3-851d-000c2940e62c"],
  "contents": { "en": "English Message" }
}

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Send to email addresses

If you have the user’s email address, you can send emails to them using the include_email_tokens property. Any emails included that do not exist within your OneSignal app will automatically create a new email subscription. 
{
  "app_id": "YOUR_APP_ID",
  "include_email_tokens": ["user1@email.com", "user2@email.com"],
  "email_subject": "Welcome to Cat Facts!",
  "email_body": "<html><head>Welcome to Cat Facts</head><body><h1>Welcome to Cat Facts</h1><h4>Learn more about everyone's favorite furry companions!</h4><hr/><p>Hi Nick,</p><p>Thanks for subscribing to Cat Facts! We can't wait to surprise you with funny details about your favorite animal.</p><h5>Today's Cat Fact (March 27)</h5><p>In tigers and tabbies, the middle of the tongue is covered in backward-pointing spines, used for breaking off and gripping meat.</p><a href='https://catfac.ts/welcome'>Show me more Cat Facts</a><hr/><p><small>(c) 2018 Cat Facts, inc</small></p><p><small><a href='[unsubscribe_url]'>Unsubscribe</a></small></p></body></html>"
}

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Send to phone numbers

If you have the user’s phone number, you can send them SMS and MMS using the include_phone_numbers property. Any phone numbers included that do not exist within your OneSignal app will automatically create a new SMS subscription. 
{
  "app_id": "YOUR_APP_ID",
  "include_phone_numbers": ["+15555555555"],
  "contents": { "en": "English Message" }
}

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Adding custom data

For personalized content, pass user-specific custom_data to the message using Templates and Liquid syntax. Steps to add custom data:
  1. Create a Template via the dashboard or Create template API.
  2. Add Liquid Variables (e.g., {{ message.custom_data.order_id }}) to your template.
  3. Reference the template_id and custom_data within your Create Message API call.
{
  "app_id": "YOUR_APP_ID",
  "include_aliases": { "external_id": ["userA"] },
  "template_id": "8458af75-4da2-4ecf-afb5-f242a8926cc3",
  "custom_data": { "order_id": 123, "currency": "USD", "amount": 25 }
}

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Example: One-Time Passcode (OTP)

  1. Identify the user using an alias, email, or phone number.
  2. Create a Template that includes a verification code:
Your verification code is {{ message.custom_data.verification_code }}
  1. Generate the verification_code on your server when the user requests access.
  2. Input the verification_code value into the API request.
{
  "app_id": "YOUR_APP_ID",
  "include_aliases": { "external_id": ["userA"] },
  "template_id": "8458af75-4da2-4ecf-afb5-f242a8926cc3",
  "custom_data": { "verification_code": "123456" }
}
Alternative: If you don’t want to use templates and custom_data you can input the variable value directly into the message with string concatenation. For example: 
{
  "app_id": "YOUR_APP_ID",
  "include_aliases": {"external_id": ["userA"]},
  "contents": {"en": "Your verification code is " + verification_code}
}

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Troubleshooting

  • For include_aliases, the alias must be registered on the user beforehand.
  • For email/SMS, ensure correct formatting.

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Additional Resources