How to send in app message to my individual user from my admin panel?

I am creating an app where users can send message to admin & after reading the message, if admin wants to send the reply to that particular user, how do I do that, please help.
I already created half of the part where user sends message to admin & working fine, but how do I send the data to particular user via firebase.
I am looking for the solution some what like this :- I as an Admin type a user mobile number in the text box & then the message in the other text box & click on Send button, then the message should directly drop into that particular users inbox list.

Use one signal extension and by using player I’d you can send message to them

I want to send the data using firebase, when I click send button the data should be stored in firebase & when that particular user login his account & open the inbox page, then the list should call the data from firebase via (if else) statement & check weather that user has any data in the fire base & if yes then show the message to user as a list item else the list remains as it is.

  1. At first create data users/user_id/inbox/message:ok
  2. Call data in list view from user inbox screen
  3. store data on specific users inbox details.