OneSignal Sent To Specific User(Push Notification) In Ionic 5

I’m new to OneSignal, How am i able to sent to specific users in ionic 5 using onesignal is there any way to receive their device token so that in the back end i able to choose those users that have press the subscribe button.
For example:

  1. User press the button (the device token will sent to one signal server)
  2. Admin able to see the user that press the button and sent them the push notification

Sorry for bad english