Create account or log in if you are already a registed user.
Click on 'Add New App' on the screen window.
You need to Create certificate or key ID for the IOS and Android.
SETUP android FCM key click here.
SETUP IOS certificate for push click here.
Once new app setup successfully, click on the newly created app and go to app-setting where you will find one signal app-id in keys/ids section.
Replace this key / id in your app.component.ts file, this.oneSignal.startInit('230d3e93-0c29-49bd-ac82-ecea8612464e','714618018341');
Here the First parameter value key you get from one signal setting and second value is Sender ID that you get from Firebase. You can click on gear icon on Overview then click on Project Setting and go to Cloud Messaging Tab and copy the Sender ID.
Now open your account window in onesignal.com and go to new-message section where you can send notification.
If you want own Admin to use to send push notification instead of oneSingnal then you can use there API to do it or you can check our backend where we already have those stuff added.