To create a task linked with a specific reservation, first you need to create a reservation note. 

See how to add reservation notes in FrontDesk Master.

Once the note is created, click on the green notepad button:

And add task details, like Title, Notes. You can assign the task to a user and choose the reminder date. Save the task.

Set From and To dates as the same date if you want to receive a notification at specific time.

You can see all created tasks in the Calendar:

See also:

  1. How to search for bookings with tasks? 
  2. How do task notifications work? 
  3. How to mark task as completed?