There are three ways to create a Key Activity that supports a milestone. Choose the one that fits your workflow best.
Option 1: From the milestone in the initiative
Go to Company and then All Initiatives.
Select the initiative and open the Milestones tab.
Click the milestone. The Milestone Details panel opens.
Under Connected Key Activities, click + Add Key Activity.
Fill in the information about the Key Activity.
Click Create.
Option 2: From the milestone in your team's Key Activities
Go to your team and then Plan Key Activities.
Find the milestone and select it.
Click Add activity.
Fill in the information about the Key Activity.
Click Create.
Option 3: When you create a Key Activity
Go to your team and then Plan Key Activities.
Click Add Key Activity.
Fill in the information. First choose the initiative, then the milestone the Key Activity supports.
Click Create.
Read more in Create Key Activity.