How to pull object with this code setup?

There are many ways to do this, but for the way you approach it (parenting the player to the box), the first step is to access to object you will move:

 MovableBox movableBox = collision.gameObject.GetComponent<MovableBox>();
 if (movableBox != null) {
     canDrag = true;
     movableBox.transform.parent = transform;
 }

However, I would go about it very differently: instead of having a canDrag variable and making the movableBox a child, just store if you collided with a movable box (in a MovableBox variable), and if you are holding ctrl and you have a MovableBox, then any movement vector should also be applied to the movable box as well (you will probably want to make the MovableBox's rigidbody kinematic).

Btw, transform and this.transform are the same thing, so saying transform.parent = this.transform; is like Darth Vader saying "Luke, YOU are your father", which doesn't make sense (it might even give you an error).