When your finally inputting the map marker

"
Navigate to stage 20 of our quest, and add an ObjectReference property to the script, and call it "DungeonMarker". Select the map marker as the target of the property.
Now, edit the stage 20 script so it reads:

SetObjectiveCompleted(10)
SetObjectiveDisplayed(20)

DungeonMarker.AddToMap()"

How do i Select the map marker as the target property?

Anyone?

After you made the objectreference property, click it then press 'Edit value'. There you can choose cell, then the reference, or you can find it in the cell viewer.