Here are the instructions to schedule a message on the app:
1. Click on the writing a message button
2. After that, tap on the Schedule button (next to the "Send" button)
3. Choose the day and the hour for your message
4. Tap on "Schedule"
5. Write your message (it can be done before the scheduling)
6. Send your message by tapping in the button "Schedule"
And voilà! You can schedule messages, it can be useful if you don't live on the same timezone than your loved ones, or if you want to surprise them!