Adding a new task in ThunderTask is very easy, you have two options to do so:
Quick add task: Add your task in the text field with the caption "Add a task and press Enter" in the header at the top of the page. To save the task, you have to press the ENTER key on your keyboard.
Add new task overlay: Move your mouse to the "New" button in the header of the app and click on "Task". This opens the "Add new task" overlay, where you can define a priority, set a due date, etc. and save your new task to the currently selected list.