Creates a PlaceholderSource object with specified values, sets this as the value of the proxySource attribute, and sets useProxy to true. It does not preserve the interpretation parameters, instead using the user preferences.
NOTE: There is no direct way to set a placeholder as a proxy in the user interface; this behavior occurs when a proxy has been set and then moved or deleted.
name A string containing the name of the new object.
width, height The pixel dimensions of the placeholder, an integer in the range [4..30000].
frameRate The frames-per-second, an integer in the range [1..99].
duration The total length in seconds, up to 3 hours. An integer in the range [0.0..10800.0].