Map markers are created by adding the component MapDescriptorComponent or SCR_MapDescriptorComponent to any entity. If added to a moveable entity then marker will move with entities movement.
If there is no entity at the position where you want to have a map marker at then you can just create a GenericEntity by using the Create-Subwindow of the world editor.
I ll create a list with the markers which are currently possible (most of them seem to be placeholders at the moment) with the MapDescriptorComponent here:
Main Type / Unit Type