Chat Notification Firebase Onesignal

I developed a realtime chat with ionic + firebase, and at the moment I'm trying to develop the notification system if a new msg will be received. I'm using Onesignal, but I have no idea how I could perform this integration.

ionViewDidLoad() {

    //recebe parametros
    this.recipient = this.navParams.get('recipientUser');
    this.pageTitle = this.recipient.name;
    //pegar remetente
    this.userService.currentUser
        .first()//somente primeiro valor
        .subscribe((currentUser: User) => {
            this.sender = currentUser;
            this.chat1 = this.chatService.getDeepChat(this.sender.$key, this.recipient.$key);//pega soemnte um chat
            this.chat2 = this.chatService.getDeepChat(this.recipient.$key, this.sender.$key);
            if (this.recipient.photo) {
                this.chat1
                    .first()
                    .subscribe((chat: Chat) => {
                        this.chatService.updatePhoto(this.chat1, chat.photo, this.recipient.photo);
                    });
            }
            let doSubscription = () => {
                this.messages
                    .subscribe((messages: Message[]) => {
                        this.scrollToBottom();
                    });
            };
            this.messages = this.messageService
                .getMessages(this.sender.$key, this.recipient.$key);
            this.messages
                .first()
                .subscribe((messages: Message[]) => {
                    if (messages.length === 0) {//se for igual a 0 nao existe msmg na lista
                        this.messages = this.messageService
                            .getMessages(this.recipient.$key, this.sender.$key);//busca ao contrario
                        doSubscription();
                    } else {
                        doSubscription();
                    }
                });
        });
}

//RECEBE MENSAGEM DA VIEW 
sendMessage(newMessage: string): void {
    if (newMessage) {//se existir msm 
        let currentTimestamp: Object = firebase.database.ServerValue.TIMESTAMP;
        this.messageService.create(
            new Message(
                this.sender.$key,
                newMessage,
                currentTimestamp
            ),
            this.messages
        ).then(() => {
            this.chat1
                .update({
                    lastMessage: newMessage,
                    timestamp: currentTimestamp
                });
            this.chat2
                .update({
                    lastMessage: newMessage,
                    timestamp: currentTimestamp
                });
        });
    }
}

//
private scrollToBottom(duration?: number): void {
    setTimeout(() => {
        if (this.content._scroll) {
            this.content.scrollToBottom(duration || 300);
        }
    }, 50);
}