You realize that dozens if not hundreds of this kind of app already exist, right? I don’t really get the point of trying to recreate apps that already are out there and work fine - specially not with a drag-drop-builder that has less possibilities and many limitations (like not being able to send push notifications at a certain time by default)…
However if you want to continue your project anyways, you might be able to do this by relying on some 3rd party that could send onesignal push messages at a certain time even when the app is closed and refer to this tutorial as well to send push notifications to certain users only: Sending Notification to Specific User with Onesignal
However it will be a lot to learn and a lot of work - therefor just as a reminder: this app already exists in several variants and I don’t see the point of doing all the work if people already use other apps for the same purpose 
Best, Chris