For the "New Task" button, click on the arrow and select "Plan-based".
Select the plan where you want to place the plan marker.
Place a plan marker at the desired location. You can also add a drawing and then click on Save.
If you want to place multiple plan markers in succession, check the "Series" box. The type is a required field and must be selected, all other task details are optional.
When you activate the slider for "All Markers", previously set markers will also become visible on the plan.