There are three types of Milestones in Planera:
Start Milestone - One is required, and this is where you set the start date of the project
Interim Milestone - You can add as many as you like, and this is to track important Interim deadlines.
Constraints available on Interim Milestone:
Start On
Finish On
Start On or After
Finish On or Before
Finish Milestone - You can add one of these to the project.
Constraints available on Finish Milestone:
Finish On
Finish On or Before
How to add Milestones to the project
Using Left Toolbar
Hover over the left Toolbar and select the Group icon.
Click on the Milestone icon, and select the type of milestone.
Drag it to the desired spot on the canvas, and release the mouse button. A new Milestone will be created in that spot.
Right-click on the Canvas
Right-click on your mouse and select Add New Interim Milestone.