Adding a milestone is the same as adding an activity in the masterplan. Click on the menu (three dots) of any parent or leaf activity that the milestone is to be added.

Click 'Add sub activity' to add a new milestone.

This is where you determine whether you want to add an activity or milestone. If you are adding a milestone, choose the duration as zero '0'. This will also prompt you that you are adding a milestone. Click Yes to confirm.

After setting both base and expected duration as 0, change the dates to whenever you want to schedule it. Then click the Tick button to add the milestone.
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You will have the milestone added successfully at the bottom of the activities under the parent activity.

Watch the video below to learn how to add or delete a milestone: