There is currently no built-in way to pass arguments to a scene like that. You have to call a function of yours probably, like this:
var scene = preload("your_scene.tscn").instance()
scene.initialize(params) # init root node
There might be only one case where
_init will be called, is when you do this:
var node = preload("your_script.gd").new(params)
But for scenes, you have to go the other way.