subscribe.kt

var db = FirebaseFirestore.getInstance()

val mPersonTable: CollectionReference = db.collection("Person")

 val person = Person(
                null,
                etName_firestore.text.toString(),
                etEmail_firestore.text.toString(),
                etPhone_firestore.text.toString())

mPersonTable.add(person)
            .addOnSuccessListener {
                        "Thanks for your subscription.".toast(this@FirestoreActivity)
            }
            .addOnFailureListener { exception ->
                "Error: ${exception.message}".toast(this@FirestoreActivity)
            }