Android Question Firebase notifications but only for a specific user?

Sub Process_Globals
    Public fm As FirebaseMessaging
End Sub

Sub Service_Start (StartingIntent As Intent)
    If StartingIntent.IsInitialized Then fm.HandleIntent(StartingIntent)
    Sleep(0)
    Service.StopAutomaticForeground 'remove if not using B4A v8+.


    dim Token as string = fm.Token
End Sub

Public Sub SubscribeToTopics
    fm.SubscribeToTopic("general") 'you can subscribe to more topics
End Sub

Sub Process_Globals
    Public fa As FirebaseAuth
End Sub

Sub Service_Create
    fa.Initialize("fa")
End Sub

Sub fa_TokenAvailable (User As FirebaseUser, Success As Boolean, TokenId As String)
    'Token id
End Sub

Sub Process_Globals
    Public fm As FirebaseMessaging
End Sub

Sub Service_Create
    fm.Initialize("fm")
End Sub

Sub fm_TokenRefresh (Token As String)
    'Message token
End Sub

Public Sub SubscribeToTopics
    fm.SubscribeToTopic(Starter.idrepartidor) 'you can subscribe to more topics
End Sub

Sub AppStart (Args() As String)
    SendMessage(4, "Esto es una prueba", "Hello!!!!")
    StartMessageLoop
End Sub

Dim strNJson As String = "{""notification"":{""title"":""EnviaPy - Nuevo Pedido"",""body"":""Nuevo Pedido de " + Trim(Session("nombre")) + "},""to"":/topics/" + idrepartidor.ToString + "}"

Private Sub SendMessage(Topic As String, Title As String, Body As String)
    Dim Job As HttpJob
    Job.Initialize("fcm", Me)
    Dim m As Map = CreateMap("to": $"/topics/${Topic}"$)
    Dim data As Map = CreateMap("title": Title, "body": Body)
    m.Put("data": data)
    Dim jg As JSONGenerator
    jg.Initialize(m)
    Job.PostString("https://fcm.googleapis.com/fcm/send", jg.ToString)
    Log(jg.ToString)
    Job.GetRequest.SetContentType("application/json")
    Job.GetRequest.SetHeader("Authorization", "key=" & API_KEY)
End Sub