How to subscribe other calendar service? (Premium)
Open TickTick on your Android device, then either swipe to the right or tap the hamburger button in the upper-left corner.
Tap the gear-shaped icon in the upper-right corner.
Tap "Security & Data", then tap "Subscribe Calendar".
Enable "Subscribe Calendar", then tap "+ Add Account", then tap the calendar you want to subscribe to or tap "Add URL" to enter a Calendar URL.