Put the object you want to spawn in a dynamic group, with PI=0. Then create a trigger that has the conditions to make it spawn (probably doctrine language, completed trigger kill x and completed trigger kill y). On the last page of the trigger, use the dropdown menu and select create group and then select the DG with your object in it.
It's possible that a mark/label on the chart can't be in a DG, but the target itself can, and you can set the target to be revealed automatically on the map.
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