How to create sub collection to firebase firestore document

Is there a way to create sub collection after set() method like shown below?

                    CollectionReference ref1 =
                        FirebaseFirestore.instance.collection('chatrooms');
                    await ref1
                        .doc(chatroom)
                        .set({"chatid": chatroom})
                        .collection('messages')
                        .add({"message": _message.text});