I was looking at an old schedule. There is a summary task in the beginning with task ID and WBS is zero. It is a task that contains all summary activities below it - actually it is not even a task, its name is project xxxxxxx. Now I am trying to make the same kind of task in the very beginning of a new schedule but I cannot. For example I tried typing 0 to ID field but it doesn't let me. I also tried changing WBS to 0, it let me but then the ID remains unchanged as 1. I am sure it was done in another way. How to do it?

Ok nevermind I found it myself. I went to Options>Advanced>Display options for this project and then marked "show project summary task"

Good for you for wanting to display the Project Summary Task, aka Task 0 or Row 0. It really needs to be displayed in every project so that we can see the overall summary for the entire project. The other method is to display the Gantt Chart view, to click the Gantt Chart Format ribbon tab, and then to select the Project Summary Task checkbox at the right end of the ribbon.

Also, you might want to take a look at one of my recent YouTube videos in which I discuss the Project Summary Task, and other important steps in initiating and planning a project at:

https://youtu.be/rRDqNh1g-qM

Hope this helps.