How to pass subscription counter to header of AlertController

Hi all
I need to show on my alert header countdown.
this.countdown is countdown every second from 10 to 0.

Here is my function

    async idleAlert() {

          this.alertIdle = await this.alertController.create({
            header: 'Countdown: ' + this.Counter,
            cssClass: 'customAlert',
            backdropDismiss: false,
            buttons: [
                {
                    text: 'Cancel,
                    role: 'cancel',
                    cssClass: 'alertBtnGrey',
                    handler: () => {
                        this.stopWatching();
                        this.authService.logout();

                    }
                }, {
                    text: 'ok',
                    cssClass: 'alertBtnSuccess',
                    handler: () => {
                        this.restartTimer();
                    }
                }
            ]
        });

        await this.alertIdle.present();

    }