How to lock the app?
There are two options to lock TickTick: Passcode and Touch ID.
Option 1: Passcode
Go to Settings page from the tab bar.
Tap "Security & Data".
Tap "Passcode & Touch ID".
Option 2: Touch ID
After you enable Passcode according to the steps above, you can then enable "Touch ID" in the same page.
Once Touch ID is enabled, the Touch ID sensor will read your fingerprint and unlock TickTick with a touch of your finger.