In order to automatically position the scene on the poster, you need to add the poster: Name to the Spot object. For example:
poster: <Main poster>
The position and rotation of the scene will be saved relative to this object when positioning.
The object must have a texture, it will be used as an image to detect the poster in the real world. The size of the object, sets the size of the poster in the real world. 1 unit = 1 meter.