How to make something with ID zero

[tropaj]

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?