You simply need to create an event that uses one of the "Pick Object" options and use that to set your target. You can then move toward it as well as firing at it. You need one event for the movement and one for the firing. Shouldn't be too hard. The movement is available for all objects, not specific to turrets. The turret behavior is simply for acquiring a target and shooting at it. You also have properties you can set for the behavior.
If you want other options like movement, then write logic to do that. Just like you would for any other type of sprite. Once you have chosen the object to target, you still have access to it's x and y position. Pick the object, then pass it to the target and move events.