From Interactive Gantt:

Go to your project. By default, you are taken to the Interactive Gantt. If not, click on the Interactive Gantt from the views.
From the Gantt menu bar, Click on the Add option and then click on Task.
This will add a new task in your project plan.
Enter your task details and click Save.

You can also add sub-tasks, Milestones, Predecessor, Successor, etc. from here.

To Add Multiple Tasks:
At times when creating your WBS, it is more convenient to add multiple tasks in one go, and then go back to schedule them. To achieve this

Go to the Interactive Gantt view of the project.
Click on the Add > Multiple.
This opens a popup where you can enter task name; one on each line.
Upon Submit, multiple tasks are created

To read about Gantt features, please click here.

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