Copy all task names from external source. Then in Project Plan 365, select the next empty task name cell and from the Task menu press on Paste or use keyboard shortcut CTRL + V / Command + V. After doing this, you will notice that the whole list of task names will be inserted for you at after selected empty task cell. The duration, start and finish date fields will be completed with default values: Duration = 1 day? Start and Finish date= Project Start date.