How can I use the Editor to set a pointer to an instanced object?

I’ve not tried to set a raw pointer this way, but I’ve had some success with setter functions.

You could try:

UFUNCTION(BlueprintCallable, Category="UnitManager")
void SetUnitManager(AUnitManager* _MgrPtr) { MgrPt = _MgrPtr;}

Then you should be able to call this on the actor and set it. Might need a corresponding getter on your other actor to grab the AUnitManager pointer.