The Postmark iOS app supports using both Touch ID and Face ID for securing the app depending on what is available on your device.
If you activate Touch/Face ID on one device, it will automatically be enabled on all devices connected to your iCloud account. If a device does not support Touch or Face ID, the device passcode will be requested instead to access the app.
Step #1: Open settings
Start by opening the settings screen.
Step #2: Enable Touch ID or Face ID
If you device supports Touch ID or Face ID, a toggle we be shown to enable the feature. You will need to authenticate with you face/finger before the setting is enabled.
To disable authentication, open settings and slide the toggle to off. Again, you will need to authenticate before the feature is disabled.