How to Create a Milestone with Project for The Web

The quickest way to create a milestone is to add a task with no duration to your project.

  1. Go to Project (project.microsoft.com) and select Timeline.Add a task to timeline.
  2. Select + Add new task, and then type a task name.
  3. Hover over the task name and select the Open details icon.Open details
  4. Type 0 in the Duration field, and then select Add dependency.Add dependency to task
  5. The milestone symbol Milestone is now part of your timeline.Milestone symbol

Ref: Add a milestone – Microsoft Support