I personally like to wake up 20-30 minutes before fajr for tahajjud so I think if the app has a feature to ask “how about tahajjud tonight?” Once the last third of the night kicks in or around 10ish because that’s when people tend to be most at rest and preparing for bed, Or something lol and then we open the app and can set the time ourselves and just like the other prayer times have a toggle to switch on and off

Jazak’Allahu khair!