When selecting an interactive object from the Behavior Properties, an object selection screen shows up.
Selecting the Desired Object
When the selection screen shows up, any object from the current layer will show up. To change the layer swipe from the left side of the screen to bring out the Layer Interface so you can select objects from different layers. When the object is selected, it will appear with a green rectangle outline with a blue target in the centre. You can then tap the checkmark at the bottom of the screen to select it.
Moving the Blue Target
In most cases, moving the target changes the anchor point of the object. You can read about behavior anchor points here. However, with some behaviors, moving the target can represent moving the objects position. An example of this is the Move By behavior. Moving the target will move the object to that position and will automatically fill in the x and y properties of the behavior. Another example would be the Rotate to Object behavior. Moving the target will determine the front side of the behavior, so that the side facing the target will point at the other object.