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In order to be able to send transactional, or user-specific notifications to your users, you will need a way to identify which device belongs to each which user.

The nativeFunctions.onesignalSetExternalUserId() function allows you to assign a custom ID to the device in OneSignal, which can be the same ID used on your website to identify users, making it very easy for you to know to whom a specific device belongs, and allowing you to send user-specific notifications to that user.

We recommend triggering this function whenever you perform a check to see if the user is logged-in, so you can assign his user ID to OneSignal accordingly, here is an example of how to implement the Javascript code:

<script type="text/javascript">
// Listen to events
window.addEventListener("message", (event) => {
}, false);

// Get the user ID
var userId = "1";

// Function that runs when the event is triggered
function runNativeFunctions(event) {
try {

// Check if the native functions are ready to be used
if ( && == 'nativeFunctionsLoaded') {
// Native function can be called here

} catch (ex) {
// Log any error messages for easier debugging

OneSignal has a nice guide with more details on how to send those push notifications using the OneSignal REST API, you can find it here:

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