Print 
PDFapp.project.item(index).replaceWithPlaceholder(name,width,height,frameRate,duration)
Description
Changes the source of this FootageItem to the specified placeholder. Creates a new PlaceholderSource object, sets its values from the parameters, and sets mainSource to that object.
Parameters
name A string containing the name of the placeholder.
width The width of the placeholder in pixels, an integer in the range [4..30000].
height The height of the placeholder in pixels, an integer in the range [4..30000].
frameRate The frame rate of the placeholder, a floating-point value in the range [1.0..99.0]
duration The duration of the placeholder in seconds, a floating-point value in the range [0.0..10800.0].