Setting 'this' when calling a function

Consider a function square and an object point:

function square() {
  return { x: this.x*this.x, 
           y: this.y*this.y };
}

var point = { x: 10, y: 20 };

If we call function square on object point in a way that this inside the function refers to point, we would get an object {x: 100, y: 400} in return. Which one of the followings is the correct way to invoke function square such that this inside square refers to object point?