One Signal is the platform used for sending push notifications to the devices with the app installed. It is free and only one account is needed to enable push notifications for all your apps. You can create an account on the OneSignal website.
Setting up an app at One Signal
Each app must have a separate entry at One Signal so its keys are unique. To add a new app to One Signal, log in with your account and click New app/Website. Enter the app name and proceed to the next step. You will need to configure two versions of the app, iOS and Android. Start by configuring the iOS version by clicking on the platform Apple iOS (APNS) and moving to the next step.
On the Apple iOS (APNS) Configuration screen you must upload the push certificate file (.p12). This certificate was created previously, together with the store certificates for iOS. If the file was created with a password, this should also be entered.
Click Save and close the modal with the cross top right
From the menu on the top, select Settings. Click the edit button next to Google Android.
The information required for Android set up can be found in on the app's entry at your Firebase account. You can follow the steps at Setting up a new app on Firebase if the app hasn't been created yet.
Navigate to Firebase and select the App. From the menu on the left, select the settings icon button and Project Settings.
Under the Cloud Messaging tab you will find the Server Key and the Sender ID (the Sender ID is the Project Number required by One Signal). Copy the numbers by clicking on the copy icon and paste them in OneSignal and save the configurations.
Integrate OneSignal with the app
To integrate OneSignal with the app, two configurations are required, one while building the app and the other one at the configurator. At the OneSignal website you can navigate to the app and choose the tab Keys & IDs in the top-right corner.
The keys must be used as it follows:
In the configurator under App Settings > Push Notifications
OneSignal App ID and REST API Key
While building the app
OneSignal App ID and Sender ID. The Sender ID can be found in Firebase, under Project Settings > Cloud Messaging.