You can create task dependencies by using blockers.
Adding a blocker will tell Motion not to schedule certain tasks before others. For example, if Task A blocks Task B then Task A will always get scheduled before Task B.
Tasks can only block each other if they are created within the same project. Additionally, you can add more than one blocker for a task.
Here is step-by-step guide:
1) Create a new project and add a few tasks
2) Once you have at least 2 tasks auto-scheduled, you can add blockers
3) Click "Add Blockers" and select tasks that prevent another task from being scheduled. In this example, my "Second Task" will always be scheduled after my "First Task".